Unit2019 pad

UNIT 2019

Know how to prepare surfaces for decoration 2

Surface preparation is very important for producing a high-quality � nish. In this unit we will look at some typical surfaces you may � nd yourself working on during your career, along with appropriate preparation tasks for each surface.

It is important that all surface contaminants such as dirt, oil, rust and loose or � aking existing coatings are removed. If these contaminants are not removed, it could affect whether the paint or paper will adhere (stick) to the surface.

This unit also contains material that supports NVQ unit QCF 332 Prepare surfaces for Painting/Decorating.

This unit also contains material that supports TAP Unit Prepare surfaces for Painting/Decorating.

This unit covers the following learning outcomes:

Preparing timbers and timber sheet products ready to receive � nishing systems

Preparing metal surfaces ready to receive � nishing systems

Preparing trowelled � nishes and plasterboard ready to receive � nishing systems

Removing previously applied paint and paper ready to receive � nishing systems

Rectifying surface conditions

Repairing and making good surfaces

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to prepare surfaces for decoration 2

Level 2 NVQ/SVQ Diploma Painting and Decorating 3rd edition

Unit 2019

K1. Preparing timbers and timber sheet products ready to receive finishing systemsTimber is one of the most commonly used materials in construction. You will encounter timber in a wide range of situations, both internal and external.

Applications of timber and timber sheet productsTypes of timberTimber is classified as either softwood or hardwood. This can sometimes be confusing as not all hardwoods are physically hard or softwoods soft. Hardwood and softwood refers to the botanical differences and not the strength of the timber. Hardwood trees are deciduous, broad leaved, with an encased seed. Softwood trees are usually evergreen with needles and seeds held in cones.

Botanical – the classification of trees based on scientific study

Deciduous – the name given to a type of tree that sheds it leaves every year

Evergreen – the name given to a type of tree that keeps its leaves all year around

Key terms

Name Appearance Properties/Description Uses

Redwood (commonly known as pine)

Moderately strong for its weight with average durability. Quality of finish depends on knots and amount of resin. Capable of smooth, clean finish and can be glued, stained, varnished and painted.

Used for interior or exterior work and for carcassing and finish joinery

Whitewood (also known as European Spruce)

Similar to redwood in strength and durability. Takes glue, nails and screws well and can produce a good finish.

Similar uses to redwood

Western red cedar

Not as strong as redwood but has naturally occurring oils which prevent insect attack. Doesn’t need treating as will stand up to severe weather and turns a silvery colour when exposed.

Externally for good-quality timber buildings, saunas, etc.

Table 19.1 Commonly used softwoods

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Unit 2019 Know how to prepare surfaces for decoration 2


Oak Very strong with English oak the strongest. Good resistance to bending and shearing. Susceptible to fungi attack and ironwork should not be used as it will stain and disfigure.

High-class joinery, panelling, doors, exposed roofing, etc.

Beech Hard, close grained and durable with a fine texture. Capable of a good smooth surface. Takes glue, stains and polish well and can produce an excellent veneer.

Furniture, kitchen utensils, wood block floors, etc.

Mahogany Strong for its weight and moderately resistant to decay. Takes glues, finishes, nails and screws well.

High-class joinery, furniture, boat building and plywood veneers

Table 19.2 Commonly used hardwoods


Plywood Made from thin layers of timber glued together to form boards. Alternating grain across and along the sheet gives strength and stability.

Used in floors, walls and roofs

MDF Made from pulped wood mixed with adhesives and pressed into sheets. Moisture resistant MDF is available.

Skirting boards and mouldings

Hardboard Manufactured from sugar cane pulp mixed with adhesives and pressed into sheets 3–6 mm thick. Has a reasonable resistance to moisture.

Flooring, furniture and units

Table 19.3 Commonly used timber sheet products

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Uses of timberTimber has several different uses in the construction industry. You will be familiar with some of these uses, particularly structural uses, from Unit 2003, pages 81–85 and page 88.

l Structural – floors, walls and roofs providing stability to the building. Timber is a core part of the construction or as decking over a framework.

l First fixing – any work inside a structure before plastering takes place. Includes studwork, ground lats, stairs, windows and doors.

l Second fixing – any work inside a structure after plastering has taken place. Includes all joinery work, such as doors, architrave, units and ironmongery.

l Decorative – any structure made entirely from timber such as fire surrounds, mouldings, balustrades, banisters, dado rails, flooring and decking/fencing.

Defects in timbers and timber sheet products, and treatmentsTimber can be affected by a wide range of defects. If you are not sure what preparation or decoration is required for a particular type of timber, you should always seek advice before starting the task.

Knotting and resin exudationA knot is a place in the timber where a branch was joined to the tree. Resin may bleed from knots, staining the paint finish. Knotting solution seals knots and can be applied to areas stained with resin, tar and ink. Its main ingredient is shellac, produced by an insect and melted into thin flakes. Clean and dry the surface before applying with a brush. It should dry quite quickly, after which time the surface coating can be applied.

Stain sealingShellac is also available coloured (known as pigmented shellac). Aluminium provides a silver pigment while titanium provides a white pigment, and these are very effective stain sealers especially on:

l stains made by fire, smoke and waterl previously creosoted timberl animal, smoke and fire odours (smells).

Resin – a natural liquid formed in wood when it is converted into useable timber. Very sticky and usually yellowish gold, under heat it emerges from knots (this is called resin exudation)

Key term

Knotting solution is highly flammable and so should not be exposed to naked flames. You must also make sure you wear the appropriate PPE when handling this material.

Safety tip

Figure 19.1 Knotting bottle

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End grain, open grain and cracksWood grain is the growth rings found inside all tree trunks. When timber is cut in the opposite direction of these rings this is called cutting ‘against the grain’ and it can cause timber to chip or tear. Other grain cuts include:

l with the grain – cutting the timber is easier and cleanerl across the grain – cutting across the grain lines but the plane

of the cut is still aligned with theml end grain – the timber is cut at right angles to the grain, for

example when trimming the ends of planksEnd grain often needs to be repaired by the decorator by stopping and filling, which should be done after priming. A stopper is stiff material used to ‘make good’ gaps and holes on surfaces and it dries with the minimum amount of shrinkage. Areas such as open joints and splits in timber need to be stopped and made flush prior to decoration.

There are several different types of filling methods that you will need to use.

l Proud filling – overfill and leave a raised amount of material filler (proud). After drying the filler will reduce or shrink back leaving it slightly proud. It will need abrading level.

l Back filling – press the material deep into the area then leave it to dry. Repeat the process until the surface is level.

l Flush filling – use a filling knife or caulk board to apply the filler and make the surface flush prior to applying coatings.

Filler used for good open-grained timberl Plastic woods – made from a mixture of resin and wood flour,

used when applying clear wood finishes. Available in a ready mixed formula and two-pack formula. Very quick setting but expensive.

l Two-pack stoppers – hard-wearing and can have fittings screwed into them. Dry very quickly and have little shrinkage. Must be used on bare surfaces as they can cause coatings to become defective due to the ingredients in the stopper.

l Putty – substance mixed with linseed oil which becomes sticky. Commonly used on wood for filling holes and to fix glass into window frames. Less expensive than other stoppers but becomes brittle with age.

Moisture contentWet rot is a growth of brown fungus in damp timber. As the fungus grows, it destroys the wood. The only long-term treatment

Open joints – gaps in timber structures

Key term

Flush – when one surface is level and even with another surface

Key term

Shellac is not only used in stain sealers. It can also be used as a safe coating on foods, such as fruit and sweets, to give them a glossy shine.

Did you know?

Figure 19.2 Stoppers

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Unit 2019

of wet rot is the removal of moisture. Before working on an area affected by wet rot, you must treat it, as outlined here.

l Rake out any defective timber. Allow the surface to dry and flood the timber with a clear wood preservative. Allow it to dry and spot prime the affected areas with wood primer.

l Fix wood screws (non-ferrous, that is non-iron) into the timber.l Apply a coat of two-pack polyester filler to the surface and

allow it to dry. The screws will help the filler adhere to the surface.

l Apply a second coat of filler and allow to dry. Abrade the filler so it is flush. An acrylic spot filler (a soft putty) may be needed to fill any minor imperfections.

Glue residue and nail headsGlue residue is excess glue left on the surface of timber. If dry, this can be removed with a scraper and then sealed with knotting solution. If wet, remove with scraper and clean with white spirit or turps and leave to dry.

Nail heads may be left projecting above the surface. Place a nail punch squarely onto the nail head, covering the whole surface of the head, and hammer the nail further into the timber until it is below the surface. Apply a stopper or filler to the hole.

Surface and physical properties of timbers and timber sheet productsTactility and aesthetics are the key characteristics of wood and timber.

l Tactility – how workable the timber is to create different structures and fittings. The more workable it is, the more potential uses it will have. Tactility also applies to other materials, such as plaster.

l Aesthetics – the finishing look of timber when it has been stained and varnished. The more pleasant the aesthetic look, the more likely it is to be used decoratively.

PorosityA porous surface is one that contains tiny holes through which liquids or gases can pass. In order to prevent porous surfaces, such as timber, from being penetrated by water or damaged by frost, a silicone water-repellent layer can be applied to waterproof the surface. When dry it is completely clear.

Why should you not put iron screws into timber affected by wet rot?

Find out

Figure 19.3 Tactility and aesthetics: the workability and beauty of timber

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Physical propertiesTimber has a number of physical properties. Some of these properties also apply to other materials you will encounter – the qualities they give will be essentially the same for each type of material.

Insulation Ability to trap heat. Wood has low thermal conductivity so it is a natural insulator. Air pockets in the wood make it a barrier to heat and cold

Hardness Resistance to wear and tear. Also difficulty to saw or plane the timber

Strength Amount of stress timber can resist before bending. The ‘stiffer’ the wood, the stronger it will be

Flexibility Ability to expand and retract without damage

Absorption Ability to absorb shocks and damage

Adhesion Ability to ‘stick’ or fix to a different surface

Table 19.4 Physical properties of timber

Capillary actionThis is a process where liquid is drawn up through a small gap between the surfaces of two materials. In a building this could allow water to rise up from the surface and into the building. Timber can be affected by this if its moisture content is above 30 per cent. Capillary action will also cause the timber to rot.

This also applies to plaster and metal areas. These will not rot but plaster will become weaker and metal will corrode (see page 111).

Appropriate abrasivesAbrading a surface means wearing away the top layer by rubbing, or creating friction. This is a very important part of surface preparation. It provides a key for the coating or covering to be applied and smoothes the surface to give a good-quality finish.

It is important that the correct type of abrading material is used.

l Abrading material that is too rough can leave scratches on surfaces that show through to the finish.

l Material that is too fine can have a long preparation time and be ineffective at removing or levelling rough surface imperfections.

l Cheap and inadequate materials (such as glass paper) can greatly extend the preparation time as they get blunt and clog very quickly.

Figure 19.4 Silicone water repellent used on porous timber

Abrading a new softwood or hardwood may result in damage due to scratching or furring (the lifting of wood fibres). For this reason, it is best to simply dust off the surface prior to painting.

Remember

Key – roughness on a surface provided to aid adhesion

Key term

Harcourt EducationJ6637 Painting andDecorating9pt Zurich BTAW036

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Abrading materials and equipment

Grades of abrasivesIt is important to select the correct grade of abrasive for each job to achieve the correct finish. The grade is printed on the back of abrasive paper and relates to the particles of aggregate to every square 25 mm.

A grade that gives a coarse abrading effect will have large particles and therefore less of them. Figure 19.5 shows aggregate on a P20 grade dry abrading paper where only 20 particles of aggregate will fit on to a 25 x 25 mm area.

A grade that gives a fine abrading affect will have lots of small particles. Figure 19.6 shows aggregate on a P80 grade dry abrading paper where 80 particles of aggregate will fit on to a 25 x 25 mm area.

Wet and dry abrasivesThese can be used in both wet and dry conditions. A waterproof adhesive fixes the abrasive particles to the backing, which means that the paper does not lose the particles when it gets wet – in fact, if wet and dry paper is used dry it tends to clog up.

The aggregates used in wet and dry abrasive paper are often silicon carbide, but aluminium oxide is becoming increasingly popular. Particles of aggregate are closely grouped together and referred to as being ‘closed coated’. Water (sometimes mineral oil) can be used as a lubricant, preventing paper from becoming clogged. Wet and dry abrasive is available in grades from P80 (coarse) through to P1200 (very fine).

Advantages Disadvantages

Extremely good for high-quality work More expensive than some dry abrasives

Wide range of grades available Unsuitable for bare timber

Cleans the surface as it abrades Clogs up easily if used dry

Low dust levels Surface must be dry before decoration

Table 19.5 Properties of wet and dry abrasives

Dry abrasivesThese use a non-waterproof adhesive to fix the abrasive particles to the backing paper. The best aggregate to use in this type of paper is aluminium oxide grit. Glass and garnet are common, but less effective.

Always choose the correct type and grade of abrading material for the surface and the job.

Remember

Figure 19.6 A large number of small aggregates will give a fine abrading effect

Figure 19.5 A small number of large aggregates will give a coarse abrading effect

Abrading will create dust particles so ensure adequate ventilation of the work area and wear appropriate respiratory PPE. Some tools are equipped with a dust collection bag.

Safety tip



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Aluminium oxide abrasive (production paper) is usually available ‘open coated’, where the particles of aggregate are spaced apart on the backing paper. This reduces the risk of clogging as the gaps allow waste to escape. Dry powder lubricants can be used on some types of dry abrasives, breaking away when heat is generated by the abrading process, preventing clogging of the abrasive. Dry abrasives are available in grades ranging from P20 (coarse) through to P320 (very fine).


When worn, particle edges shear off revealing smaller but sharper edges (Figure 19.7)

Aluminium oxide can be expensive compared to other abrasives

Available in sheet, roll, disc and belt form High dust levels produced

Available in self-adhesive rolls – abrasive can be torn off and fixed to purpose-made rubbing blocks

Table 19.6 Properties of dry abrasives

Mechanical sandingElectric tools can greatly reduce time spent preparing surfaces and increase the surface area covered. Electrical sanders work by moving an abrasive pad or belt at a fast speed.

Belt, drum and orbital sandersThe heavy duty sanders most commonly used by a decorator are belt, drum and orbital sanders.

Belt and orbital sanders are hand-held power tools best used for sanding large, flat items of joinery. A drum sander is self-propelled and used for stripping floors. A rough grade of abrading material should first be used to remove surface coating. The surface can then be brought up to a smooth finish by progressively using finer and finer abrading material.


Effective at abrading large areas More expensive than abrasive papers

Faster rate of abrasion than by hand Only suitable for large, flat areas

Can create large amounts of dust

Table 19.7 Properties of belt and orbital sanders

Figure 19.7 How aluminium oxide breaks down

Figure 19.8 Belt sander

Figure 19.9 Drum sander


New abrasive

Aluminiumoxide particleswear down andbreak away

Remainingparticles aresmaller andsharper

Figure 19.10 Orbital sander

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Disc or rotary sandersRotary sanding involves the use of rotating discs of abrasive material and can be used to prepare small or contoured surfaces.

Different types of abrasive disc are available:

l flat discs that require a backing padl flap discs made up from flaps of abrasive, which are more

expensive but also more effectivel grinding discs that can be used for removing very heavy, small

areas of rust.


Do not burnish the surface Only suited to small areas

Effective at removing isolated patches of rust

Not suited to complex surfaces (discs cannot reach into awkward corners)

Relatively low initial cost of equipment

Table 19.8 Properties of disc or rotary sanders

If the sander is equipped with a dust collection bag, make sure it is working and empty it before using the tool. After sanding, the wood dust collected should be disposed of appropriately. Sanding dust should not be left in bags indoors as there is a danger of it catching fire through its own heat.

Appropriate solvent-based and water-based primerPriming is the first coat of paint applied to a surface. Primers protect the substrate and give an even and consistent finish to the final coat. Universal primers are designed to be used on a range of surfaces. If the surface preparation or the application and choice of the primer is incorrect in any way, the durability of the paint system will be reduced.

l Solvent-based primers – form a waterproof layer to prevent wet rot in timber surfaces. They do not raise the grain of the wood when applied, but have a longer application time than water-based primers as the solvent content prevents them from drying as quickly as water-based primers.

l Water-based primers – provide a moisture screen to the surface. Water-based primers do not soak into timber, meaning they adhere less well than oil-based paints. They are low in VOCs and odour, quick to dry and more durable on hardwoods.

Figure 19.11 Disc (or rotary) sander – in this instance, an electric drill fitted with an abrasive disc attachment

Small electric sanders are also available with triangular heads for use when sanding corners.

Abrasive discs can be fitted to electric drills and angle grinders.

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Primer Description

Acrylic primer Water based. Reapplied as an undercoat to speed up coating process. Mostly used for internal timbers but can be used externally.

Aluminium wood primer Solvent based. Has aluminium non-leafing particles to make it more suitable for priming resinous timbers such as Columbian and Oregon pines. Can be used for both internal and external timbers.

Wood primer Solvent based. Can be used on softwood and hardwood internal and external timber surfaces.

Preservative primer Solvent based. Used for external timbers only. Similar to wood stains and varnishes as it protects the timber

Table 19.9 Types of primer

Correct preparation process for rectifying defects in timberThere are different preparation techniques for each type of timber surface. Follow the techniques described earlier in this unit, as well as remembering the different qualities of types of timber, before you begin work.

Bare untreated timberFor basic painting tasks:

l seal any knots in timber using knotting solutionl prime the surface using oil-based wood primer (for external

surfaces) or acrylic primer undercoat (for internal surfaces)l fill using polyfiller and decorator’s caulk, rub down and dust offl apply one coat of undercoat, rub down and dust offl apply another coat of undercoat if necessary then apply one

coat of glossAlternatively, for staining or varnishing tasks:

l fill holes in timber with putty or coloured stopper and apply a base coat

l rub down and dust offl apply one coat of wood stain or varnish and lightly rub down

and dust offl apply second coat of wood stain or varnish.

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Previously painted timberl Rub down using sandpaper. If necessary, fill using polyfiller

or caulk.l Apply one coat of undercoat, rub down undercoat and dust off.l Apply one coat of gloss.

Rough cut timberRough sawn timber should be dry brushed thoroughly to remove soil, vegetation and dust.

l Apply one coat of timber preservative or wood stain.l Apply a second coat of timber preservative or wood stain.

K2. Preparing metal surfaces ready to receive finishing systemsMetal has a wide range of uses throughout buildings and structures. It can be used for frames, girders and trusses. Some metals, such as aluminium, are also used for decorative reasons. There are several different types of metal that you may encounter as you work. Metals are either pure or alloys.

l Pure – the metal is made from only one element. Common examples of this include gold, silver, lead, copper, aluminium, zinc, iron and tin.

l Alloy – a mixture of two or more metal elements, used when a strong, light metal with properties that do not exist in a pure metal is needed. Aluminium alloy is a common example of this.

Surface and physical properties of metal typesMetal shares some qualities with timber, including porosity (page 102). The other key qualities of different types of metal are shown on page 109. Metals will not exhibit all these qualities.

Element – a substance that cannot be broken down into any other substance. For example water is not an element as it can be broken down into hydrogen and oxygen. These cannot be broken down so are elements

Key term

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Quality Description

Malleability Can be hammered and pressed into different shapes

Conductivity Has the ability to conduct heat and electricity

Elasticity Can regain its shape after being misshapen during use

Hardness Resistant to scratches and cuts during construction

Brittleness Extremely hard but can be broken very easily

Ductility Can be stretched without breaking and turned into a fine wire. Metals with this quality are used for metal cables.

Toughness Can absorb shock and energy without breaking (the opposite of brittleness)

Tensile strength Tested under extreme conditions (pulling, squashing, twisting and shearing) and can withstand these forces

Heaviness Denseness of the metal in relation to its size

Strength Tested to withstand heavy loads without breaking

Table 19.10 Key qualities of different types of metals

Types of metal and their applications

Non-ferrous metalsNon-ferrous metals do not contain iron and are not magnetic. They are usually more resistant to corrosion as they have non-friable oxide layers, created by the atmosphere. These should be dry and free from grease prior to painting. Previously painted non-ferrous metals need to be abraded and any corrosion deposits found should be scraped back to a firm edge where any flaking paint is evident.

Ferrous metalsFerrous metals contain iron and may have small amounts of other metals and elements added to them to give them the properties they need. Most ferrous metals are magnetic. These surfaces are prone to rusting and will need to be cleared of all rust prior to painting. Depending upon the extent of the rust, it can be removed with the use of a wire brush, mechanical wire brush, abrasive papers and/or scrapers.

Use the Internet as well as materials lists and manufacturers’ information to find out more about the circumstances where these qualities of metals are particularly desirable.

Find out

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Table 19.11 Non-ferrous and ferrous metals


Copper(non-ferrous)

Extremely ductile, malleable, good conductor. Tarnishes and oxidises quickly. Easily damaged and must be stored carefully.

Available in tubes, sheet, wire, rod and flat bar. Used for water pipes, electrical wiring and roofing.

Aluminium(non-ferrous)

Extremely malleable, ductile, lightweight and conductive. Non-toxic and often alloyed. Highly resistant to corrosion. Can be dyed (anodising).

Some types of window frames. Excellent for stamping and forming.

Lead(non-ferrous)

Very soft, malleable, heavy and highly resistant to corrosion. Tarnishes to a dull grey when exposed to air. Very poor conductivity. Poisonous, so care must be taken.

Roofing

Galvanised steel(ferrous)

Highly resistant to corrosion, as alloyed with zinc to protect the iron. Can withstand saltwater, moisture, rain, snow, etc. Lightweight, fire-resistant, basically maintenance free and extremely durable and resistant to scratches and abrasion.

Girders, frames, roofing, support beams, piping, etc. Available in tubes, sheets, ropes and flat bar.

Cast iron(ferrous)

Corroding metal, non-toxic, made by melting pig iron and small amounts of scrap steel. Strong, hard, self-lubricating and brittle but also cheap, well wearing and sustains heat.

Bridges, buildings, stairs, handrails, cast iron columns, items such as machinery parts

Wrought iron(ferrous)

Iron alloy with very low carbon content made by melting porous iron with slag and other impurities. Gives it properties not found in any metal.Tough, malleable and ductile. Can crack if bent or heated up and brittle when cold. Has a rough texture so it can hold platings and coatings.

Roof trusses, ornamental ironwork, pipe work, handrails. Available in bar form, sheets, rods and hoops.

Mild sheet/steel(ferrous)

Iron alloy, corrodes and has high carbon content so vulnerable to rust. Malleable, ductile and tough with high tensile strength and bends easily.

Girders, tubes, screws, nuts and bolts and garage doors (use composites for wood apppearance).

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CorrosionDuring your inspection of the work surface, you may notice areas where the surface has corroded, usually due to rust. This will have to be cleaned and removed before work can be carried out.

Main corrosion factorsCorrosion is the destructive attack on a metal from its environment. The main corrosion factors are all found in the atmosphere. They are:

l oxygenl hydrogenl moisturel pollutants.The most common form of corrosion is oxidation. The atoms in metal combine with the atoms in oxygen to form oxides. Iron rust is the most recognisable form of corrosion and is caused by iron oxide appearing on iron or steel components. Metals with high iron content will corrode more than metals with low iron content. Metals which show signs of corrosion, such as pitting, must be repaired and the metal protected with a coating.

Corrosion can protect some metals, such as copper. Copper is used as both a pure metal and an alloying material. Because it is very resistant to corrosion, it does not need to be treated with protective coatings as most other metals do. It develops a protective oxide coating. This thickens the copper and turns it green.

Galvanic corrosion and cathodic protectionElectrolysis is an electrochemical process, where a metal comes into contact with an electrolyte (a conductor, usually water) and parts of the atoms of the metal (electrons) flow from the metal into the electrolyte, causing it to corrode. This is called the galvanic action. If two metals are in the same environment, the metal that has less resistance to electrolysis (the anode) will corrode before the other metal (the cathode). Some metal coatings have metal in them lower than the actual substrate being painted. This provides protection to the substrate, as the anode in the coating causes it to corrode first. This is known as cathodic protection.

Anodising – an electrochemical process that converts metal surfaces into a decorative, durable, corrosive-resistant, anodic oxide finish. Aluminium is the metal often used, but titanium and magnesium can also be used. These metals are immersed in an acid electrolyte bath with an electric current running through the medium

Pitting – formation of small pits in a metal surface as a result of corrosion. Crevice corrosion occurs where nuts, bolts and gaskets have been used

Key terms

Use the Internet and other resources to try and find some examples where electrolytic corrosion is used.

Find out

On iron and steel a thin, flaky black iron oxide called millscale can form (see page 113). This is an example of cathodic protection to the steel.

Did you know?

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To use an example from Table 19.12, if iron sheet has a zinc coating, this will protect the iron as the zinc will rust first. The zinc is less resistant than the iron and therefore is the anode.

Copper

Tin

Lead

Nickel

Iron

Zinc

Aluminium

Magnesium

Table 19.12 Metals in order of their resistance to corrosion

Appropriate primers and their function

Primer Description

Mordant solution Also known as etch primer or T-wash. Very toxic and used for non-ferrous metals. Must have an overcoat after application to avoid deterioration. Available in two-pack coatings, which have better stability and adhesion but a limited shelf life once mixed. Touch dry within 1–4 hours and can be recoated in 10 to 14 hours.

Metal primer(acrylic-based)

Specially formulated to prevent rust and provide adhesion, low VOCs, non-toxic, odourless. Expensive to purchase and can only be used on properly prepared surfaces. Can be recoated in 4 to 6 hours.

Zinc phosphate Solvent-based primer with a rust-inhibitive pigment, touch-dry within to 3 hours. Suitable for non-ferrous metals, iron, steel and blast-cleaned surfaces.

Table 19.13 Primers and their functions

Preparation processes for metalSolvent wipingSolvents (for example, white spirit and turps) remove grease and oil from metal prior to decoration. Solvents are used to avoid rust on the surface. Solvents are very toxic so make sure you are in a well-ventilated area and that you are wearing the correct PPE.

Blast cleaning – an alternative way to remove corrosion and coatings from steel or metal work. It involves grit particles being shot through a hose under high air pressure, which removes everything it hits on surfaces. This method is used in heavy industry and in fabrication shops.

Key term

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Removing rust by handCleaning off rust by hand is normally done when repainting rusty steelwork, as it is usually the cheapest method. The problem with hand cleaning is that the use of scrapers, chipping hammers, wire brushes and abrasives will not remove all traces of rust. In addition, the overuse of a wire brush can serve only to polish the rust on the surface, affecting the ability of the primer to adhere to the surface.

Follow this procedure when cleaning by hand.

l Remove any traces of oil or grease to avoid spreading it around the surface.

l Scrape off all loose rust, millscale and previous coatings.l Use a chipping hammer around rusted nails, bolts and rivets.l Use a wire brush to remove loose rust, but avoid burnishing.l Finish off by abrading with a rough aluminium oxide abrasive –

P40–P60 (see page 104–106).

Removing rust with power toolsPower tool cleaning is generally quicker and more effective than hand cleaning and will extend the life of the paint system. Loose rust, millscale and the existing surface coating can be removed using power wire brushes, grinders and needle guns, although some millscale will not be removed even with power tools. Again, care should be taken not to over-polish the surface or the adhesion of the primer will be negatively affected.

Follow this procedure when cleaning with a power tool.

l Remove any traces of oil or grease from the surface.l Scrape off all loose rust, millscale and previous coatings.l Use a needle gun to remove rust around corroded nuts, bolts

and rivets, etc.l Select the most effective method of removing rust to suit the

nature and condition of the surface (for example, rotary wire brush, disc sander or angle grinder).

Steelwork l Dry abrade using emery paper or a scraper and wire brush, and

dust off.l Apply good general purpose metal primer or zinc phosphate

to areas where rust has been removed (apply a full coat for previously painted steelwork).

l Apply undercoat.l Lightly abrade, dust off and apply a gloss coating.

Millscale – a thin flakey black iron oxide formed on iron and steel. It is also a cathodic protection to the steel

Burnishing – polishing

Key terms

Figure 19.12 A wire brush can be used to remove loose rust

Figure 19.13 Power tools such as needle guns and angle grinders can be used to remove rust from surfaces

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Other metal surfaces

Ferrous metals (iron and steel)l Remove all corrosion and millscale via mechanical means.l Degrease with white spirit if necessary.l Allow the surface to dry thoroughly and apply primer with

a brush.l Bitumen-coated surfaces will require sealing with shellac

knotting solution or aluminium primer.l Four coats of paint will be required to achieve adequate

film thickness (as recommended by the British Iron and Steel Association).

Non-ferrous metals (aluminium, copper, zinc, brass etc.)l Degrease surface with white spirit.l Galvanised and zinc-sprayed surfaces should be treated with

mordant solution.l Etch the surface with wet and dry abrasive paper and white

spirit to provide a key.l Apply one coat of metal primer or universal primer.

K3. Preparing trowelled finishes and plasterboard ready to receive finishing systemsPlasterboard is a durable and high-quality lining for walls and ceilings, lift shafts and stairwells, corridors and auditoriums. Trowelled finishes are surfaces constructed from bricks and blocks. Most walls use bricks and blocks as a central part of their construction.

Bitumen – a heavy, semi-solid, brown-black substance created as a result of the oil refining process (also known as asphalt or tar)

Key term

Jamila has given a client an estimate for repairs to discoloured, flaky and rusty metal railings. She states that the railings can either be removed and abrasive blasted, then treated and a full paint system applied or they can be prepared by hand and spot primed before a paint system is applied to them. There is a cost difference between the two because of the timescales involved.

Which system should be chosen? What has caused the deterioration of the metal railings? How would the metal be treated after being blast cleaned? What suitable paint system could be used? Which estimate should the client go ahead with?

Working life

Plasterboard can also offer extra moisture protection as it has moisture controlling and water-resistant properties.

Did you know?

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Types of surface and applicationsPlasterboardMost plasterboard is made from gypsum, processed into a board and given a paper covering. Standard plasterboard is suitable for most applications and is compatible with direct decoration or plaster finishes. It has a grey facing and an ivory coloured back. Plasterboard is used in a range of residential and commercial buildings.

Dry lining is where dividing walls in a building are made from plasterboards (square and feather-edged) which are attached to timber structures (stud walls) and taped up. The joints of the two boards are sealed with joint tape with the joint then plastered over for a smooth finish.

BlockworkConcrete blocks are heavy but produce strong finished work. They are used where a lot of weight will be put on top of, or against, the wall. They are also used to form footings below ground on walls that support steel. Lightweight blocks are lighter versions of concrete blocks, produced in response to health and safety restrictions on lifting and handling units heavier than 20 kg.

Block Appearance Uses

Solid block Concrete block used for making walls above ground for commercial, industrial and leisure buildings. Also used for beam and pot floors.

Hollow block Concrete block used where reinforcement is needed. Same finish as solid blocks but with hollow sections running through them. Filled with vertical reinforcement rods and concrete, making them very strong.

Aircrete block Lightweight block made of a microcellular composition (aircrete). Lightweight but very durable. Used for foundations, beam and block floors and internal and external cavity walls.

Table 19.14 Types of blocks and their uses

Figure 19.14 Plasterboard

Gypsum – a white rock produced as the by-product of industrial processes

Key term

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BrickworkBricks are smaller than blocks and so more are required per square metre. A brick is 215 mm long, or half the length of a block, and 65 mm high. The length of a brick is called the stretcher and the end of a brick is called the header.

Like blocks, bricks are held in place by mortar, a mixture of sand, cement and water used for bedding and jointing. Bricks, like blocks, need to be laid level and straight with equally sized joints to achieve a sound wall with a good appearance.

Physical properties of plaster and trowel surfacesPlaster, brickwork and blockwork share a number of physical properties with metal and timber. Plaster has qualities of capillarity, tacility, adhesion and porosity. Brickwork has qualities of capillarity and porosity.

Property Description

Acidity Found in some surfaces and can cause defects if the surface has not been prepared and primed correctly

Inertness Refers to plaster being able to bond to surfaces without reacting or causing defects

Soluble salt content Salt found in brickwork. It must be removed correctly or will keep returning and lead to efflorescence (see page 117)

Table 19.15 Some physical properties of plaster, bricks and blocks

AlkalinityThe chemical nature of surfaces such as concrete, cement rendering, asbestos sheeting and some plasters is alkaline. This can cause problems if a solvent-based paint is applied as the alkalinity in the surface can attack the paint, causing saponification.

To prevent this, you should apply an alkali-resistant primer. This forms a barrier between the surface and the paint. The permeable nature of acrylic surface coatings means they need an alkali-resistant primer to prevent alkalinity coming through if the surface becomes damp.

Alkaline – having a pH greater than 7 (an acid has a pH of less than 7)

Saponification – a chemical reaction that makes soap and so foams up as a result

Permeable – allowing things to pass through

Key terms

Figure 19.16 Alkalinity permeating through an acrylic coating


Alkalinitypermeatingcoating

Acryliccoating

Alkalisurface

Client: HarcourtJob No: J6598Fig No: AW078

65 mm

102.5 mm

Header

215 mm

Stretcher

Figure 19.15 Sizes of brick, header and stretcher

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Defects associated with plaster and trowel workLike timber, plaster and trowel work can suffer from cracks, nail heads and open joints. Moisture must also be removed from plaster and trowel surfaces to avoid mould growth and many of the defects covered below.

Settlement cracks, shrinkage and dry outSettlement cracks appear in plaster and cement work in floors and ceilings. They are caused by shifts in the elevation of a structure caused by shifts in the soil. The soil may not have been compacted properly before construction and, as it decays, collapses – leaving a void beneath the building.

Shrinkage is caused by material drying out after completion and cracking as it shrinks. In cement, if too much water is added to the mix then it can shrink once this water has evaporated. Plaster that has been properly adhered will avoid shrinkage.

EfflorescenceEfflorescence can be seen as the white patches on cement-based surfaces and it can occur on brickwork and plaster. Because cement is porous, moisture can penetrate it, dissolving some of the lime and creating calcium hydroxide. This then rises to the surface when the cement dries out, leaving white patches of calcium carbonate.

Efflorescence will have to be removed before decoration. Scrub the surface with a stiff fibre brush or a wire brush. Never remove efflorescence by washing the surface as the calcium carbonate will dissolve in the water and sink back into the cement.

Defective rendering and raking outRendering is a coating of plaster applied to stonework. Cracks can form in this.

To repair small cracks:

l scrape away any loose coatings and particles of masonryl apply filling agent – exterior grade filler (polyfiller type) could

be used but this would probably re-crack after a short period of time, whereas exterior acrylic caulking will provide more permanent flexible repair.

Figure 19.17 Settlement cracks

Figure 19.18 Efflorescence

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When dry the mortar can be painted over, but this repair would probably re-crack after a short period of time.

Alternatively, rake out, undercut and fill as above and then allow to dry. Apply a bituminous caulking compound over the crack and bed a nylon type bandage over the length of the crack. Further applications of the caulking compound can then be made over the bandage to provide an invisible reinforced repair, which will last longer.

PoppingPopping occurs when movement causes plaster to break away from plaster nail heads. To rectify:

l dry scrape and dust offl spot prime the nail head and apply the correct filling

agent proudl when the filler is dry, rub down to leave an even surface and

apply a coating.

If a hole or crack on a surface is not filled proud it can shrink when drying and will need filling all over again.

Remember

Repairing a large crack in rendering

Step 1: Rake out any loose coatings and rendering using a 25 mm scraper or pointing trowel.

Step 2: Undercut the crack. Step 3: Fill the crack with a sand and cement mortar to a ratio of one part cement to three parts soft sand. This is known as ‘pointing up’.


The crack mustbe undercut togive the mortarholding power


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Processes for rectifying surface defectsSome of the processes for rectifying surface defects have been covered earlier in this unit; abrading (page 103), filling (page 101) and degreasing (page 112).

Wetting inWhen removing wall coverings from plasterboard wet in first. Apply water to the wallpapered area after first scoring the paper. This water penetrates the paper making it softer and easier to remove. You can also wet in holes and cracks in rendering before applying filler to prevent it drying out before it sets.

ScrapingScrapers can be used for many different tasks such as removal of wallpaper, rust, nibs from plaster surfaces and flaking paint and pastes. Defects must be scraped off before you can begin painting.

CaulkingCaulk is a waterproof filler and sealant used in cracks and gaps. Mastic is an acrylic type of caulk and is applied using a mastic gun, which is a frame that holds and helps dispense mastic from its tube. When it is dry, mastic feels a bit like rubber.

Surplus caulk can be removed with a filling knife. Any remaining material can then be sponged off. Figure 19.21 shows two examples of how caulking can be applied to the tops of skirting boards. The illustration on the left shows the caulk applied correctly. The illustration on the right shows incorrect application.

Appropriate primers for trowelled finishes and plasterboard

Primer Description

Emulsion When used as a primer it is thinned down with water and used on porous surfaces. Also used to seal plasterboard before applying further coatings as it helps to stop coatings being absorbed.

Primer sealer Solvent based with good alkali resisting properties designed to seal porous, dry and friable surfaces and touch dry within 8–12 hours. Cannot be recoated until 16–24 hours later and should not be thinned to stop loss of properties. Can also be used to seal defects.

Alkali-resisting primer (ARP)

Solvent based and specially designed for surfaces that are alkaline and require a solvent-based finish. New, dry interior plaster and masonry finishes best primed with this coating. Touch dry after 8 to 12 hours and re-coatable after 16 to 24 hours.

Table 19.16 Primers and their uses

Figure 19.20 Caulking with a mastic gun

Correct Incorrect

Figure 19.21 Correct and incorrect caulking

Figure 19.19 Scraping knife

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Preparing surfaces according to finish requiredPlaster

Untreated plasterboardl Apply one coat of emulsion thinned by up to 10 per cent.l If necessary, fill using polyfiller then lightly rub down and

dust off.l Apply one coat of emulsion as an undercoat, rub down and

dust off.l Apply second coat of emulsion.

Bare plasterl Dry scrape with a scraper or broad knife.l Apply one coat of alkali-resisting primer or one coat of

emulsion thinned by up to 20 per cent.l Fill any holes or dents using polyfiller then rub down using

sandpaper and dust off.l Apply first coat (eggshell or emulsion), rub down and dust off.l Apply second coat of paint (eggshell or emulsion).

Previously painted plasterl Wash down using sugar soap solution.l Fill holes or cracks using polyfiller and decorator’s caulk then

rub down and dust off.l Apply first coat (eggshell or emulsion), rub down and dust off.l Apply second coat of paint (eggshell or emulsion).

Masonryl Clean the surface with a jet wash or scrub with a suitable

detergent, remove loose materials and treat any efflorescence.l Any mould should be treated with a sterilisation wash, also

known as fungicidal wash, before being removed with a scraper. The surface should then be re-treated with the sterilisation wash.

l Ensure surface is completely dry before applying any coating.l Prime new masonry and older or weathered masonry with

stabilising solution or all-purpose primer, applying with a brush. Previously painted surfaces in good condition may not need priming. Subsequent coatings can be applied by brush, roller or spray.

Newly plastered walls can be painted with emulsion because it allows the wall to ‘breathe’.

Did you know?

The reason the first layer of primer applied to masonry should be applied with a brush is because the action of brushing forces the paint into the surface.

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K4. Removing previously applied paint and paper ready to receive finishing systemsIf the surface you are working on already has a coating of paint that is in poor condition (for example, it has a brittle paint film or paint actually flaking off), it will be necessary to remove the entire coating in order to produce a good finished effect.

Paint and paper defects that need to be removedSome of the defects that can lead to paint and paper being removed have been covered earlier in this unit. Information about peeling paper from plasterwork can be found on page 119. The following defects must be removed prior to redecoration to prevent them showing through new coatings and coverings.

Defect Description

Blistering Blisters on wallpaper are caused by trapped air. Blistering on paint is caused by trapped moisture or by painting on hot surfaces.

Cracking or crazing Paint film which has become thick with layers of paint can become excessively hard and brittle with age, causing cracks. To rectify fully prepare the surface and abrade.

Excessive film thickness

Caused by applying too much paint to a surface, spoiling the final appearance. A liquid paint stripper or blast cleaning will be needed to remove.

Peeling Caused by lack of adhesive on wallpapers or wear and tear on painted surfaces.

Table 19.17 Defects that must be removed before redecorating

Alice is working on a barn conversion with an apprentice, Michael. The client wants the outside painted cream and brown. Because of time pressure, Alice tells Michael to start applying cream emulsion straight onto the walls while she applies brown emulsion to the woodwork.

Should Michael do this? What paint system should be used? What defects will occur if the paint system is not right? What should be done first to any of the surfaces? What paint system could be used on this project?

Working life

You will need to protect the work area before beginning work – turn to Unit 2020, pages 134–137 for more information on protecting the work area.

Remember

In answering the questions in this section you are practising the functional skills required to read different texts and take appropriate action, e.g. respond to advice/instructions, FE 1.2.1 – 1.2.3 This may also involve giving oral answers to questions from you tutor and is practice for speaking and listening – FE 1.1.1 – 1.1.4

Communicating effectively is an essential skill in everyday work. You may be expected to take part in discussions about your work and asked for your opinions.

Functional skills

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FlakingFlaking is caused by paint film splitting, causing hairline cracks to appear in the paint, which in turn leads to the flaking of paint chips from the surface. It can be caused by the coating being either over thinned or spread too thinly over the surface. To rectify this problem, remove the loose or flaking coating with a scraper or wire brush then abrade the area affected. You will need to prime any bare spots then repaint again.

MouldMould is a furry growth of micro-organisms (a fungus) that grows in moist or warm conditions. If mould is found, all traces of it must be totally removed to prevent it re-establishing underneath an applied coating. The following procedures should be followed to remove mould growth.

l Wet the mould to avoid the spread of spores to other areas.l Remove heavy patches of mould with a scraper or wire brush.l Apply a fungicidal wash to the affected area and allow it to dry.l If possible, the affected area should be left for a week and

re-treated if the mould reappears.In most cases, only one application will be necessary. This is because fungicidal wash has a residue effect on the surface, which means that traces of it remain, continually removing mould growth from the surface, sometimes for many years.

Removing paint from substrates and safety precautions when doing soThere are different methods that can be used to remove paint from substrates before any redecoration or applying of new coats. You will need to be aware of a number of safety issues when using these methods.

Decontaminating surfaces following use of liquid paint removersAfter using liquid paint removers you need to remove all traces of it by washing the area down with warm water and detergent before flushing the surface with clean water. This will prevent damage occurring to other surfaces, as well as accidents such as burns. Any residue of the liquid paint remover will also react with future coatings.

Figure 19.22 Flaking

Figure 19.23 Mould growth

Fungicidal washes are poisons and should be treated with extreme care. PPE must be worn at all times – rubber gloves, goggles and suitable overalls. After using this type of product, make sure that you wash your hands before eating or touching food.

Safety tip

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Removing paint with heatRemoving paint by burning it off with heat, using either an LPG (liquefied petroleum gas) burning off torch or a hot air stripper, is the fastest method of removing coatings from timber surfaces.

LPG burning off torch


Fast and efficient method of removing thick layers of paint

Many local authorities have banned LPG because of fire risks

Can be used when there is no electrical supply

Danger of cracking glass in windows when working around frames

Low running costs Scorches timber easily.

Table 19.18 Advantages and disadvantages of using an LPG burning off torch

Some important safety notes when using an LPG burning off torchl When starting up, check the hose and fittings for gas leaks

using a solution of detergent and water.l Ensure a fire extinguisher is nearby.l Avoid burning off any timber adjacent to the roof structure

of a building, as there is the risk of igniting any birds’ nests or denatured timber. You should also remove all curtains and furnishings when burning off around window frames.

l Always cease burning off operations at least one hour before you leave site and always carry out a final check for smouldering timber just before you go.

l Make sure the area is well ventilated to allow any smoke or fumes to escape.

Hot air strippersWhereas an LPG burning off torch uses a naked flame, a hot air stripper, as the name suggests, uses hot air to heat the paint, which can then be scraped off.

Removing paint with chemicalsBoth water-based and solvent-based chemical paint removers soften the paint coating, which can then be removed using hand tools such as shave hooks and scrapers.

Water-based paint removerWater-based paint remover is the one most commonly used in the trade and is available in a gel form.

Figure 19.25 Hot air stripper

Always wear the appropriate PPE when using chemical paint remover (gloves and goggles).

Safety tip

Some local authorities have banned the use of LPG strippers in favour of hot air strippers. This is because there is a reduced risk of fire and damage to property.

Did you know?

Figure 19.24 LPG burning off torch

Paint fumes give off toxic gases when burned so make sure you are wearing the correct PPE.

Safety tip

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Used as an alternative to burning off in areas where fire risk is high

All traces must be removed before scraping off the paint to avoid damaging future coatings

Doesn’t scorch or damage the surface Slow, messy and expensive

Can be used on most types of paint Can soften some plastic surfaces

Can raise the grain of timber

Chemicals and fumes can be harmful to health

Table 19.19 Advantages and disadvantages of water-based paint remover

Solvent-based paint removerThis is very good at removing thick layers of paint (up to 3 mm thick) from many different types of surface such as fibrous plaster, timber, stone, marble, brick and cast iron. The paste should be applied thickly to the surface (between 3 and 6 mm thick) with a trowel or filling knife. It can then be covered with cling film or greaseproof paper to prevent the solvents from evaporating and improve their action (sometimes referred to as a poultice).

The paint remover can be left to act on the surface for periods ranging from two hours to five days, depending on the type and thickness of the coating being removed:

l thin layers of paint – two to three hoursl thick layers of paint – overnightl very thick layers of paint – two or more daysl ornamental mouldings may have to be left for up to five days.


Same as for water-based paint remover Expensive and time-consuming

If left for correct period of time, no scraping is necessary and the paint remover can be washed off, making it ideal for delicate surfaces that cannot be scraped

All traces must be removed from the surface and the surface neutralised with water or white spirits to stop chemicals from continuing to work

Table 19.20 Advantages and disadvantages of solvent-based paint remover

Steam stripperUsing a steam stripper is a very efficient way of removing surface coverings from both walls and ceilings. Care must be taken when using this method as over-application of the steam process can result in damage to the covered surface, leading to blistering and/or removal of small areas of plaster finishes.

Figure 19.26 Steam stripper

As with hot work, make sure you conduct a risk assessment and be aware of the risk of burns and scalding from the steam and hot air.

Safety tip

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Working with pre-painted and pre-papered surfacesPre-painted surfacesBefore painting over an already painted surface, you should carry out a simple test to find out how well the old layer of paint is adhered to the surface. If it is not strongly adhered to the surface, there is little point painting over it as your finish will not last very long. The ‘scratch test’ can be performed on most surfaces as part of the preparation process.

Pre-papered surfacesWhen removing wallpapers from surfaces you may come across both vinyl papers and embossed papers that have been painted over. To remove vinyl wallpaper, peel away the vinyl film from the backing paper. This usually comes off in large strips. Then you can either leave the backing paper on the surface as a lining paper or remove it by soaking and stripping it with a scraper.

There should be a starting point for soaking wallpaper. It is best to soak the paper a few times before scraping, as the longer the soaking time, the easier it is to remove the paper.

If there is a starting point, this will allow the last area where water is applied to soak sufficiently – meaning all the areas have had the same soaking time. This will speed up the task.

Figure 19.27 Removing wallpaper

If hand soaking wallpapers for removal, you must use rubber gloves or gauntlets, similar to when washing down. You will also need to make sure that any electricity in the room is turned off, in order to avoid the risk of electrocution.

Safety tip

The scratch test

Step 1: Use a sharp trimming knife to make a few cuts in one direction and a few cuts in the opposite direction

Step 2: Cover the scratches with a piece of masking tape, pressing down firmly. Quickly rip the tape from the surface. The adhesion of the paint is poor if it comes off on the masking tape.

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Removing painted wallpapers is more time-consuming because the paper has a coat of paint. Score the paper then apply water in stages until the paper becomes soft enough to remove with scrapers.

Stripping paper infected with mouldIf wallpaper is infected with mould, you must remove these deposits. Soak the area with a mould solution or use household bleach (one part) mixed with water (up to six parts). Scrape off the contaminated paper then apply an anti-mould solution to the area before redecorating.

Storing painting tools and equipmentSome of the storage issues for paint tools and equipment were covered in Unit 1001, pages 12–15. More information on brushes and rollers can also be found in Unit 2020, page 137. Electric hot air guns and steam strippers should be stored in cases in a lockable dry storage area. Other equipment, such as metal containers and non-combustible panels, should be stored at a low level in a suitable storage area.

Other storage issues:

l Abrasive papers – store in packets on shelves with their grade clearly identified. Keep them away from excessive heat as this makes them brittle.

l Dust sheets – store on shelving and folded neatly. Must be stored dry to avoid mildew and rot.

K5. Rectifying surface conditionsThroughout this unit we have looked at some of the common surface conditions you will come across when working as a painter and decorator. This section looks at some additional conditions, defects and rectification processes which you may encounter.

Moss and lichenThis is a form of algae usually found on brick, stone and block walls. It spoils the appearance of surfaces and damages their structure, for example loosening mortar and causing brickwork to spall. To rectify, apply a moss and algae killer to the infected area. Leave this in place for the recommended time (usually about 30 to 60 minutes). Then wash the area down to remove the dead growth and treat with a suitable moss and algae prevention solution to prevent the spores from returning.

Figure 19.28 Correct storage of a variety of tools and equipment

Spall – when brickwork breaks up, flakes and becomes pitted

Key term

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Friable surfacesA friable surface crumbles away easily when you rub your hand over it. Examples of this kind of surface include weathered cement rendering or old, spalled brickwork. Paint applied to these surfaces will also crumble off. A stabilising solution can be applied. Brush the surface down with a stiff brush to remove loose particles. Then apply the solution, which soaks deep into the surface acting like a glue, binding it down.

Rectification processes and defectsMany of the common rectification processes are covered elsewhere in this book including scraping (page 119), abrading (page 103), brushing (page 113), washing down for a finish (page 128) and face putty (below).

Chalking and powderingThis is a fine powder created on a coating by weathering. Heavy chalking leads to film erosion and is caused by low grade, highly pigmented paints or by using interior paints on exterior surfaces. Remove chalk by scrubbing with a stiff bristle brush, or using a power washer, and then rinse the area.

Wrinkling or shrivellingThis is a rough, crinkled paint surface caused by contaminated surfaces, previous coatings that have not dried correctly, hot conditions or exposure to rain, dew, fog or high humidity before drying. To rectify, scrape or sand the substrate then recoat the surface, making sure conditions are correct and each coat dries fully.

Defective puttyWhen removing old paint from window frames, some of the putty is likely to break away. After surface preparations have been completed, the bare timber can be primed and any defective putty replaced with linseed oil putty.

Any old putty that has not broken away will be firmly adhered to the window frame and will not need to be replaced. However any gaps between the old putty and the glass must be completely sealed by forcing in linseed oil putty using a putty knife.

Figure 19.30 Defective putty


Glass

Linseedoil putty

Defectiveputty

Timber

If you are working on a bonded asbestos surface, make sure you take precautions and wear the correct PPE, for example, gloves and a face mask.

Safety tip

Figure 19.29 Paint applied to a friable surface


Paint is removed alongwith the crumbly partsof the surface

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Other surface conditions

Table 19.21 Surface defects and causes

Cleaning surfacesWhen washing down a surface, it is very important that the correct washing agent is used. Dirt can be removed with sugar soap or a mild detergent. Oily and greasy marks will probably only come off with the use of white spirit or turps applied with a cloth or brush. Wax polish will need to be washed off with a suitable solvent or warm water and sugar soap. You may also find that smoke needs to be removed from surfaces. Make sure that the area is thoroughly rinsed after cleaning and allowed to dry completely.

When cleaning a surface you will need to know what the surface coating is. Wipe the surface using a clean rag with a stain remover. If the rag is still clean, the surface is solvent based. If it is dirty, the surface is water based.

When washing a surface, you should always start at the bottom and work upwards. This avoids streaking of painted surfaces, which can damage the finish.

Did you know?

Surface condition Appearance Description/Cause

Cissing Coatings applied on contaminated surfaces, which prevent adhesion. Allow paint to dry before using wet and dry abrasive paper to abrade the surface. Wash down with warm water and detergent then rinse.

Bittiness Small particles of dirt on a wet coating. Abrade the surface then dust down and remove dirt before repainting.

Runs, sags and curtains Too much coating applied to a surface and paint running down. Do not overload brush and spread out coating.

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K6. Repairing and making good surfacesThis unit has discussed how the different types of surfaces – timber, brickwork, blockwork, plaster, cement and plasterboard – can be affected by defects, and the methods of correcting these defects.

It is pointless to prepare and paint a surface if it is damp or if weather conditions are wet or very cold as this will affect the paint finish or its ability to dry. These are environmental considerations and must be taken into account during surface preparation.

Wet, unseasoned timber or wet, newly plastered or washed walls must be given an adequate drying out period. Applying any surface coating before drying could lead to blistering, peeling, discoloration and staining.

Decorating in cold conditions (below 5°C) or wet weather can result in:

l failure of water-based paints to dry (due to lack of adhesion)l washing off of water-based paintsl blooming of alkyd finishes (for example, loss of gloss and a

cloudy surface)l rain pitting of alkyd finishesl peeling.

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Check it out

1 Prepare a report explaining the differences between

softwoods and hardwoods and explaining where

you may encounter them when working. You

should make reference to the properties and

possible defects that each type of wood may have.

2 Prepare a method statement, with diagrams,

explaining how to use abrasives for timber surfaces.

3 Explain, with diagrams, each stage that may

occur during corrosion, focusing on the process

of electrolysis. Explain the advantages and

disadvantages this process may have.

4 Complete a method statement explaining the best

methods for removing rust from metal surfaces.

Your statement should explain how to prepare these

surfaces for the application of a primer.

5 When spot priming metal surfaces, name two

different primers used and which metals they would

be applied to. Explain why these primers would

be used.

6 Describe the different types of trowelled and

plasterboard finishes you may encounter when

working and explain some of the qualities of each.

7 Explain how defective rendering may occur and

what can be done to rectify this.

8 Using diagrams and photographs, collect examples

of the main paint and paper defects you may

encounter while working. Explain how these arise

and what can be done to rectify them.

9 Prepare a method statement explaining how to

remove paint with heat, including reference to the

health and safety practices that must be followed to

ensure safe working.

10 Explain the purpose behind cleaning surfaces before

work and state the best tools and equipment to use

for these.

11 Explain the potential problems that can arise when

plaster is affected by heat and moisture.

FAQ

Can I use caulk to fill holes in surfaces?

No! It is not advisable to use caulk to fill holes and

cracks because you cannot abrade caulk when it dries.

This is because its flexible properties do not let it harden

off. Caulk is only recommended for filling the tops of

skirting board and around architrave to seal any gaps.

How long do you leave a steam stripper on a surface when removing wall coverings?

It depends on how many layers of wallpaper are on

the surface. However, you should not allow the steam

stripper to stay in any one place too long as you can

cause the plaster on the surface to ‘blow’. This is when

the plaster lifts off the surface therefore creating a

much bigger defect that needs repairing.

Why do you have to ‘wet in’ prior to applying fillers to holes and cracks?

You ‘wet in’ before applying fillers to holes and cracks

to help with the adhesion of the filler and to remove

any excess dust and debris that has not been fully

removed during the preparation stage.

When removing rust and millscale from metal, is it not better to make sure that you totally remove all traces and make the surface clean and shiny?

Although the rust has to be removed from the metal, it

is not advisable to make sure that the surface is clean

and shiny as, when you try to apply the paint system,

the primer will not be able to adhere to the surface as

there will be no key for the paint to stick to.

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The information contained in this unit, as well as continued practical assignments that you will carry out in your

college or training centre, will help you in preparing for both your end-of-unit test and the diploma multiple-choice

test. It will also aid you in preparing for the work that is required for the synoptic practical assignments.

When painting and decorating you will need to be able to prepare a range of surfaces to receive coatings. This will

include not only new surfaces made from a range of materials, but also surfaces that have previously been painted

or decorated. To carry this out you will need to know the techniques and methods used to prepare these surfaces,

as well as repairing and making good any defects or damage to the surface before work.

You will need to be familiar with:

• preparingtimbersandtimbersheetproductsreadytoreceivefinishingsystems

• preparingmetalsurfacesreadytoreceivefinishingsystems

• preparingtrowelledfinishesandplasterboardtoreceivefinishingsystems

• removingpreviouslyappliedpaintandpaperreadytoreceivefinishingsystems

• rectifyingsurfaceconditions

• repairingandmakinggoodsurfaces

Most of these learning outcomes revolve around learning about the implications of working with different surfaces,

and dealing with the defects that could occur as you work with them. For example, for learning outcome three

you will need to select the correct processes for rectifying the defects you have learned about when working with

plaster, plasterboards, brickwork and blockwork. You will also need to select the correct preparation processes for

these surfaces, using the correct tools, equipment and materials to ensure a high-quality finish. Using all of this you

will need to prepare the surface to receive the finish, ensuring that you are working safely and being

environmentally aware.

The knowledge you have gained about the different types of surfaces, and the preparation techniques needed for

each, will prepare you for any aspect of the practical test where you will need to prepare or repair or surface.

These same skills will be vital throughout your career as a painter and decorator.

Before you carry out any work, you should set out a plan of action, which will tell you the order in which you need

to do things. It will also record a rough timescale for the work you need to carry out, in order to make sure that

you complete everything you need to do safely. You will need to refer back to this plan at each stage to make sure

that you are not making any mistakes as you work, or missing out any part of the process that you need to work

through. Without checking this you could make some serious mistakes that could have an impact on the final build.

Your speed in carrying out any tasks in a practice setting will also help to prepare you for the time set for the test.

However, you must never rush the test! Always make sure you are working safely. Make sure throughout the test

that you are wearing the appropriate PPE and using tools correctly.

Good luck!

Getting ready for assessment

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Know how



Unit 2019

CHECK YOUR KNOWLEDGE

1 Which of the following is not a classification of

wood?:

a hardwood

b softwood

c gypsum

d timber sheet product

2 Why is knotting applied to softwood structures?

a to prime knots in them

b to undercoat knots in them

c to seal knots in them

d none of the above

3 When using fillers what should be done before use?

a area should be dusted

b area should be wetted in prior to applying filler

c area should have loose materials removed

d all of the above

4 What is the best material to use to fill gaps around

the tops of skirting boards and around frames?

a caulk

b stopper

c filler

d all of the above

5 When washing wall surfaces prior to decoration

where is the best position to start?

a the top

b the bottom

c the sides

d anywhere on the wall

6 Which grade of abrasive paper would you class

as coarse?

a P20

b P80

c P220

d P320

7 Which primer is used on galvanised metals?

a universal primer

b mordant solution

c metal primer

d zinc phosphate primer

8 Which of these describe dry lining?

a plasterboards fixed to stud walling

b plasterboards fixed to solid walls

c plasterboards not used

d none of the above

9 After using paint stripper on a surface how do you

decontaminate the area?

a wipe down with a cloth

b dry it off

c wash it down

d sand it off

10 When removing vinyl wallpapers from surfaces it is

best to what?

a score the paper first

b wet the paper first

c peel the vinyl first

d all of the above

Unit2019 pad

Design

Transcript of Unit2019 pad