NAFI Datafile FM1 (2004) - Exterior Finishes For Timber.pdf

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    Exterior

    Finishes forTimber

    Revised Edition2004

    Timber Manual

    Datafle FM1

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    Contents

    Introduction......................................................... 2

    Factors Effecting Performance.......................... 3

    Substrates ............................................................3

    Design and Construction .................................... 4

    Surface Preparation............................................ 4

    Finish Types .........................................................5

    Film Forming versus Penetrating Finish......... 5

    External nishes .............................................. 7

    Permeability ....................................................7

    Wood Primers .................................................. 7

    Undercoats....................................................... 8

    Finishing Coats................................................ 8

    Gloss................................................................ 8

    Semi-Gloss/Satin............................................. 8

    Flat/Matt and Low Sheen ................................ 8

    Natural Finishes...............................................9

    Semi-Transparent orGrain Enhancing Stains................................... 9

    Opaque Stains, Solvent Borne.........................9

    Opaque Stains, Water Borne ........................... 9

    Clear Exterior Finishes.................................... 9

    Application of Coating...................................... 10

    Maintenance ...................................................... 11

    Specications ..................................................... 11

    Introduction

    Generally, the function of nishes on timber is two-fold

    to improve the durability of the building and to add to

    the aesthetics.

    Unnished, unprotected timber will inevitably weather

    as a result of gradual changes to its physico-chemical

    structure brought about by temperature and moisture

    content variations. The rate is generally slow, at about

    6mm per century.

    The weathering process leads to a slow breaking down

    and wearing away of surface bres, change in colour and

    roughening of the surface. Under extreme conditions,

    timber may deform, check, split and pull away from

    fasteners. The extent of weathering will vary with timber

    species and ambient conditions.

    Although unprotected timber has been used externally

    for centuries, the weathered effect is not always

    desirable. In most applications timber needs protection

    from the elements of water, cold, heat and ultra-violet

    light to promote a long service life. In addition to

    protection from the elements, nishes may provide a

    decorative effect.

    The performance of exterior nishes depends on a wide

    range of factors that are covered in this Datale. The

    local environment however, is a variable which needs

    special consideration and local experience can be of

    assistance. Advice should be sought from manufacturers

    or Timber Advisory Services whose contract details are

    provided on the back page of this Datale.

    The selection and correct application of the most

    appropriate nishing product is a most important

    component of the design and construction process.This Datale provides guidance on the various

    nishes available and the applications for which they

    are best suited. It is essential, however, that after a

    product has been selected, the product manufacturers

    recommendations are followed in respect of timber

    preparation, product application and material

    maintenance.

    Cover Photo: Opaque nish adds colour and

    protection to plywood cladding

    Weather

    exposed

    framing

    protected and

    decorated withopaque nish

    blending with

    bush setting.

    Opaque nished

    timber screens

    light

    colour reects

    heat.

    This revised edition of Timber Manual Datale FM1 was supported

    in part with funding from the Forest and Wood Products Research &

    Development Corporation (FWPRDC).

    The Corporation is jointly supported by the Australian forest and wood

    products industry and the Australian Government.

    The information, opinions, advice and recommendations contained

    in this Datale have been prepared with due care. They are offered

    only for the purpose of providing useful information to assist those

    interested in technical matters associated with the specication and

    use of timber and timber products. While every effort has been made

    to ensure that this Datale is in accordance with current technology,

    it is not intended as an exhaustive statement of all relevant data, and

    as successful design and construction depends upon numerous factors

    outside the scope of the Datale, the National Association of Forest

    Industries Ltd accepts no responsibility for errors or omissions from

    this Datale, nor for specication or work done or omitted to be done

    in reliance on this Datale.

    NAFI 1989, 2004

    ISBN 1 86346 019 5, ISBN 1 86346 021 7 (set)

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    Factors Effecting Performance

    The performance of external nishes is a signicant

    factor affecting the service life of a building. However,

    fashion, new technologies, aesthetic improvements and

    changes in function often necessitate modication of

    the building, including its external envelope at more

    frequent intervals.

    In addition to these changes, the durability of an

    external nishing system on timber or timber products

    is inuenced by a range of physical factors including

    those listed in Table 1. Reference should also be made to

    Datale P4 -Timber-Design for Durability.

    Substrates

    The properties of timber, plywood, laminated veneer

    lumber, glued laminated timber and other wood based

    building products are dealt with in detail in other

    Datales in this series.

    Timber is essentially a variable substrate, even withina single species. The density, moisture content,

    absorbency, exibility and the nature of its extractives

    may vary considerably. Some timbers are also more

    susceptible to dimensional change due to moisture

    or humidity variations. The nishing system must be

    sufciently exible to cater for this movement.

    Differences in paint performance over earlywood (spring

    growth) and latewood (summer growth) can occur

    particularly on softwoods where latewood is generally

    harder, smoother and darker than earlywood. Paints with

    a tendency to embrittle on ageing (that is, those that are

    solvent borne) may ultimately crack and peel away from

    the harder surface despite having good initial adhesion.

    Uniformly ne textured timbers have better paint

    holding characteristics than coarse textured species.

    The latter, however, respond better to penetrating stain

    applications than conventional paint systems.

    The manner in which timber is cut from a log affects

    the orientation of the annual rings in the piece and this,

    in turn, affects its paint holding properties. Back-sawn

    softwood boards have wider, darker bands of latewood

    than quarter-sawn boards, the latter tending to hold paint

    better. In hardwoods the difference is comparatively

    small compared to that of softwoods.

    If a board is quarter-sawn (edge-grained) as opposed

    to back-sawn (at grained), its dimensional stability is

    better under variable moisture conditions and it is less

    likely to result in surface checking to the detriment of

    the paint system.

    It should be noted however, that most parcels of timber

    are supplied as a mix of back-sawn and quarter-sawn

    pieces. Finishing systems are developed to cater for this

    variation.

    Discolouration or blistering of the nish may be caused

    by water-soluble extractives of the heartwood such as

    gum or resin. This effect is accentuated by heat and

    is more prevalent on the side of a structure receiving

    maximum sunlight. Kiln drying of the timber will

    usually set these extractives, removing or minimising

    the problem.

    Some timbers have a higher percentage of aromatic oils,

    which contribute to their characteristic pleasant odours

    but may present problems with paint discolouration

    and inhibit drying if left unprepared before painting.

    Cleansing with a solvent may be necessary.

    Other timber characteristics that affect paint

    performance are knots, bark, gum pockets and insect

    damage. Knots will in most cases absorb more of the

    nish than side-grain and have a greater potential to

    check and exude resin or gum.

    Where plywood or glued laminated timber is used, an

    exterior timber grade must be specied together with aconventional nishing system for timber.

    Exterior grade hardboard has a smooth uniform surface

    nish and is usually supplied pre-primed, ready for

    nishing.

    Table 1: Factors effecting performance of nishes

    Semi-

    transparent

    timber stains

    enhance

    resorts

    residential

    complex

    Factor Effect on Performance

    Substrate Refer Table 2.

    Environmental

    Conditions

    Dry temperate conditions favour very

    good performance compared with

    tropical, marine and severe industrial

    environments which provide

    relatively severe exposure conditions.

    Pre-treatment Use of preservative impregnation or

    water repellent preservative treatment

    before painting, together with putties

    and sealants where appropriate, will

    improve service life.

    Maintenance

    Schedule

    Re-coating before major paint

    lm breakdown occurs, results in

    improved durability at reduced cost.

    Application

    Procedure

    Adequate lm build and attention to

    coating end-grain results in improved

    performance.

    Building

    Design

    Protection from elements using roof

    overhangs, verandahs, pergolas,

    etc., and avoiding water traps will

    promote improved performance.

    System choice Correct choice of a system for

    each particular job and use of one

    manufacturers products in each

    system will contribute to the long

    term performance of the nish.

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    Design and Construction

    The two major factors which affect the performance

    of the nish, and subsequently that of the timber, are

    moisture and sunlight. Any design features which

    minimise possible moisture ingress to the substrate

    or exposure to high angle incident sunlight, will be

    benecial.

    In Australia (Southern Hemisphere) the north facing

    exposure is most severely affected by sunlight, with the

    severity of exposure changing with the latitude of the

    site itself, varying from 15 degrees near Cape York to

    about 45 degrees near Tasmania. Guidance on the design

    of structures to provide shade protection is given in NSB

    120, Shadow Angles.

    Some design steps which can be taken to assist in

    excluding moisture and protecting from U.V. are:

    the use of wide eave overhangs, verandas, pergolas

    etc.

    use of vertical rather than horizontal siding

    use proles with rounded arrises t drip caps over doors and windows

    Avoid or minimise joins in horizontal sidings likely

    to allow moisture ingress

    provide adequate ventilation of roof space and sub-

    oor space

    use the recommended nail size and pattern for

    various types of timber cladding

    selectively use boards that show defects, such as

    knots. When you do, use them for the least exposed

    areas.

    Surface Preparation

    The purpose of preparing a substrate for painting is to

    ensure that it will accept and retain the nish with the

    minimum of interference from surface contaminants or

    from surface deterioration of the substrate itself. The

    prole of the surface and its porosity will also inuence

    adhesion of the paint system and its ultimate durability.

    For new timber, the prole can vary from a smooth

    (planed) surface to a sawn textured surface. This will

    Substrate

    Variable

    Effect on Performance

    Species Differs between and within species.

    Density The heavier, denser timbers are lessprone to moisture uptake than lighter

    species

    Dressed timber Better performance with conventionalpaint systems.

    Sawn timber Better performance with solid andnatural stains.

    Timber edges Sharp edges create stress in paint lms,causing failure. Arrised or rounded

    edges permit lm movement with

    minimal stress.

    Back-sawn

    timber versus

    Quarter-sawn

    timber

    Quarter-sawn softwood boards

    generally have better dimensional

    stability than back-sawn, and therefore

    better paint holding properties. For

    hardwood the difference is minimal.

    Unseasoned

    (green) timber

    Greater likelihood of blistering and

    peeling occurring particularly with

    solvent borne coatings. Stains are

    more effective.

    Seasoned timber Best condition for painting.

    Weathered

    versus

    unweathered

    timber

    Surface deterioration of timber bres

    due to weather can result in poor paint

    adhesion unless corrective sanding is

    carried out. Stains may perform better

    than conventional nishes if sanding is

    omitted.

    Extractives May cause topcoat discolouration or

    blistering unless surface extractives areremoved with a solvent wash prior to

    priming.

    Heartwood

    versus Sapwood

    Heartwood has a higher staining

    potential than sapwood but is ideally

    suited to painting with exterior timber

    stains.

    Earlywood

    versus Latewood

    Cracking on latewood is more likely

    to occur as the solvent borne paint

    embrittles on extended exposure.

    Smooth texture

    versus coarse

    texture

    Smooth texture surfaces have better

    paint holding potential than coarse

    texture ones. Use of stains on the

    latter gives good performance.Gum pockets Can lead to resin exudation and

    staining unless pre-treated and sealed.

    Aromatic oils Can lead to drying retardationand staining if surface oils are not

    removed.

    Knots Possible premature cracking, stainingand resin exudation can occur unless

    treated with knotting varnish or

    manufacturers recommend treatment.

    Bark Can lead to premature failure of alllm-forming nishes if not removed.

    Insect Damage Premature failure may occur if nottreated or lled before nishing.

    Moisture

    Content

    Normal range 10-15% - higher levels

    detrimental, particularly for solvent

    borne nishes where moisture vapour

    permeability is low.

    Table 2: Substrate effect on performance

    Knots and sawn

    surface texture

    show through

    semi-transparent

    timber stain.

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    dictate the nal appearance when coated.

    The sawn or roughened (coarse sand papered) surface

    will give a more positive mechanical key to applied

    coatings but, in the case of a conventional paint system,

    will result in a much thinner lm on the surface peaks

    which will deteriorate more rapidly. Some surfaces are,

    however, ideally suited to opaque or semi-transparent

    stains.

    Timber with excessively high moisture or resin/aromatic

    oil content should undergo further seasoning to rectify

    the conditions before painting.

    As recommended by the paint manufacturers,

    surface contamination by dust, grease, oil, plaster or

    mortar droppings, brick-cleaning solutions, or other

    substances likely to affect paint adhesion, must be

    thoroughly cleaned before priming. Prevention of such

    contamination is better than subsequent cleaning.

    A special consideration exists with buildings adjacent to

    the sea, where salt contamination of the surface occurs

    very rapidly and is not always visible. This must be

    washed off with fresh water shortly before the start of

    work, while still allowing time for the surface to dry. If

    prevailing winds are carrying salt laden air to the site,

    repeat washings will be required each day painting is to

    proceed.

    For timber which has been exposed to the elements

    for a considerable time and has suffered the weathers

    oxidative effects on both lignin and cellulose in the

    surface layer, the timber should ideally be sanded or

    dressed to a fresh surface before priming. Alternatively

    the use of penetrating type stains, after a simple brush

    down, will give good results.

    Any holes or depressions in the surface which mayhave occurred as a result of mechanical damage or are

    natural defects such as resin pockets, knots, surface

    splits or checks should be scraped clean, primed and

    lled with putty compatible with the paint system to be

    used. Damaged or decayed sections should be cut out

    and replaced. Adjacent timber should be treated with

    a fungicide or water repellent solution. The source of

    moisture giving rise to conditions conducive to decay

    should also be located and the fault rectied.

    As with design, emphasis must be placed on preventing

    moisture ingress, particularly where end-grain is

    concerned. Adequate sealing, with sealants, water

    repellents or primers is essential.

    Some timbers and wood based building boards are

    supplied pre-primed and provided they are not exposed

    to the elements for extended periods, may be nished

    with undercoat and an enamel paint or two coats

    of an acrylic paint without further priming, (or as

    recommended by the manufacturer).

    Veranda roof

    provides

    protection to

    external walls.

    The preparation for repainting a previously painted

    surface will depend largely on the condition of the

    surface. If the paintwork is aking, blistering, cracking

    or checking, it will be necessary to remove either the

    entire lm back to the substrate or at least enough of it

    to provide a sound base for subsequent coats.

    If the surface is chalking, or has surface mould present,

    manufacturers directions for removal should be

    followed. This is particularly critical where water borne

    systems are being used.

    Finish Types

    There is a wide range of nish types with selection

    being governed by the degree of protection required and

    the appearance sought. Table 3 summarises the various

    nishes.

    Table 4 provides guidance on the selection of nishing

    systems for various applications, but advice is also

    available from manufacturers and reference can be made

    to the standard AS/NZS 2311 Guide to the Painting of

    Buildings.

    Film Forming versus Penetrating Finish

    All nishes provide a coating on the surface and to some

    extent ll voids in the microscopic surface structure.

    Film forming and penetrating nishes vary in thickness

    and in the nished shape of the surface.

    Film forming nishes appear as a distinct layer and

    usually display a plane surface. Penetrating nishes on

    the other hand follow the contours of the timber surface,

    providing thicker deposits in the troughs and thinner

    coatings over the peaks. Refer to Figure 1.Due to the greater coating thickness, lm forming

    nishes are more resistant to wear, but penetrating

    nishes have the ability to accommodate differential

    movement of timber due to moisture variations. The

    Water repellent-

    containing

    preservatives

    can provide

    protection from

    moisture but

    require regular

    maintenance.

    Sawn nished

    plywood provides

    a good base for

    surface nishing.

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    Paint Type Comments

    Conventional opaque systems -

    water and solvent borne

    Best suited for use on dressed and seasoned timber. Wide colour ranges with

    use of tinting systems, good durability.

    Natural timber nishes - semi-

    transparent and opaque

    Best suited for use on sawn or textured durable timbers. More frequent but

    much simpler maintenance.

    Water Repellent Preservatives Temporary pre-treatment to prevent soiling and deterioration during

    construction but have limited life if not overcoated.

    Wood Primers Brush application is recommended. Solvent borne better penetration,

    but slower drying. Water borne better long term exibility, quicker

    overcoating and easier clean up.

    Undercoats Good opacity and bridging properties. Increase system lm build and

    durability. Solvent borne best over chalky surfaces. Water borne faster

    drying, better colour qualities and easier clean up.

    Finishing Coats - Gloss Solvent borne have sharper gloss, good ow, opacity and durability. Water

    borne have slightly lower initial gloss, better gloss retention, poorer ow

    but better lm exibility after aging. Water borne system applied to primed

    timber should prove superior durability to solvent borne.

    Finishing Coats - Semi-Gloss Comments as for gloss but gloss level lowered. Slight reduction in durability

    but minimise appearance of surface imperfections.

    Finishing Coats - Flat and Low

    Sheen

    Solvent borne rarely used for exterior. Water borne good durability and

    colour.

    Natural Finishes - Opaque or Solid

    Stains

    Generally solvent borne. Enhance both timber grain and texture and help

    maintain timber colour. Care required in application to avoid lap joint colour

    difference.

    Natural Finishes - Semi-

    Transparent Stains

    Obscures grain but enhances texture. Have better durability than semi-

    transparent stains. Water borne acrylic solid stains generally give superior

    performance to solvent borne nishes particularly over knotty timbers.

    Natural Finishes - Clear Enhances timber appearance but requires a commitment to more regular

    maintenance with frequent inspections. Solvent borne clears have improved

    with use of U.V. absorbers specic to resin type used. Water borne clears,or slightly tinted versions, have durability somewhat equivalent to opaque

    systems. Water borne clears are more exible and thermoplastic.

    Table 3: Exterior nishes and applications

    Finishing Systen Cladding Decking Joinery Framing

    H

    ardwoodand

    C

    ypressPine

    S

    oftwood

    H

    ardwoodand

    C

    ypressPine

    S

    oftwood

    H

    ardwoodand

    C

    ypressPine

    S

    oftwood

    H

    ardwoodand

    C

    ypressPine

    S

    oftwood

    1. Primer + undercoat + minimum 2

    coats solvent borne or water borne

    paint

    2. Primer + minimum 2 coats solvent

    borne or water borne opaque stain

    3. WRP + 2 coats solvent borne stain - - - -

    4. Minimum 2 coats solvent borne

    or water borne opaque or semi-

    transparent

    - - - -

    5. Minimum 3 coats clear exteriornish

    - -

    - -

    Table 4: Finishing Recommendations

    Notes:

    1. This table is provided as a guide only to those nishing systems suitable for the specied applications.

    2. Stains are more successful when applied to sawn nishes.

    3. Clear exterior nishes have a relatively short service life and require regular maintenance when used in a fully exposed situation.

    .

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    advantages of penetrating nishes include:

    natural appearance

    do not peel or blister

    suited to sawn textured surfaces

    do not trap moisture in timber

    are easily applied and renewed

    External nishes

    Many pigmented nishing systems for both protection

    and decoration consist of a three-stage process:

    primer

    undercoat

    If a nish is not applied on all surfaces of a piece

    of timber, differential moisture changes may cause

    distortion. Such problems may be overcome by

    providing balanced moisture permeable coatings on all

    surfaces.

    Finishing coats and the nishes used for external

    applications can be divided into two broad categories:

    solvent borne

    water borne

    Permeability

    Most coatings are impervious to water but none are

    completely impervious to water vapour. Surface lms

    are superior in this regard but after initial cracking or

    peeling, protection is dependent on the nish that has

    penetrated into the wood. Penetrating nishes may

    therefore be more effective.

    Solvent borne nishes are more resistant to water vapour

    than water borne nishes, and where a high level ofprotection is desired, a solvent borne system should be

    selected. Where recommended by the manufacturer, a

    solvent borne primer and/or undercoat system may be

    used under a water borne nishing system to provide

    Figure 1: Finish types

    improved water vapour protection.

    Wood Primers

    Wood primers provide good adhesion to the timber and a

    good base for inter-coat adhesion of subsequent coats.

    They may be solvent borne, comprised of drying oils

    and/or synthetic alkyd resins and can incorporate zinc

    oxide or other fungicidal pigment to assist in fungal

    control of the system.

    The more recently developed, water borne primers,

    containing an acrylic emulsion as the principal binder,

    are an alternative which offers good adhesion to

    timber and greater exibility, particularly on ageing.

    The problem of staining, when applied to timbers

    containing natural, water soluble tannins, has been

    largely overcome by use of xative pigments and special

    emulsions developed for this purpose. Water borne

    systems allow greater water vapour permeability than

    solvent borne systems.

    Current wood primers are lead free and are generally

    tinted to a pink colour. This provides greater apparent

    opacity and a contrasting colour to the next coat being

    applied. It avoids areas being missed during application

    and ensures that a sufcient second coat is applied to

    adequately cover the primer uniformly.

    Note that timber treated with light organic solvent

    preservatives (refer AS 1604 Specication for

    preservative treatment) may be delivered in a pre-

    primed condition. This should not be mistaken for

    properly primed timber ready for top coating. Pre-

    primers are low cost, solvent-borne, quick drying,

    factory applied products, which are not formulated to

    provide long service life and exibility of nish.

    Undercoats

    These are the bridging coats between the primer or

    previous paint surface and the nishing coat. They have

    Weather exposed

    structure and

    joinery to have

    ends and joints

    sealed against

    water penetration.

    Primingprovides

    protection to

    timber and a

    good base for

    nishing coats.

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    good bridging properties across cracks, good brushability

    under warm windy conditions, facilitate sanding, good

    gloss hold out, enhance the durability of the total system

    and have good opacity that is capable of substantially

    obscuring the background colour or colours.

    Similar to primers, they can be solvent or water borne.

    Solvent borne undercoats are generally superior for

    adhesion if applied to slightly chalky or powdery surfacesand can be formulated to give superior sanding properties.

    Water borne undercoats have better colour and non-

    yellowing qualities. They are faster drying with much

    shorter times before overcoating. They may be over-

    coated with either water or solvent borne coatings, subject

    to manufacturers recommendations.

    Finishing Coats

    In all pigmented coating systems, (better known as paints,

    as distinct from stains), the water borne paints have a

    decided advantage over solvent borne paints in terms of

    ease of clean up of equipment and faster drying and re-coat times. One restriction, however, is that they must not

    be applied or allowed to dry at temperatures below 10

    degrees Celsius, as lm formation may not occur and the

    paint may remain powdery.

    Manufacturers directions should be followed at all times,

    particularly with regard to the system or succession of

    coats to be applied.

    Gloss

    These are available in a very wide range of colours

    in both solvent and water borne coatings. The need to

    provide a wide colour range, while restricting stock items,has led to the use of tinting systems in preference to large

    numbers of ready mixed colours.

    Most solvent thinned gloss enamels can be used for

    interior as well as exterior use, but certain colours may

    be restricted by the manufacturer to interior use only

    because of the poor light fastness of certain colours. Most

    solvent borne exterior enamels are formulated with alkyd

    resins and have largely replaced the traditional linseed oil

    house paints. They also contain small amounts of acrylic

    polymers to provide the nish with some exibility. They

    have a sharp gloss, good opacity and exterior durability.

    Water borne gloss exterior nishes are now widely used.

    They have superior chalk resistance, exibility and gloss

    retention. They frequently provide a useful lm life of

    double that of the solvent systems, if they are applied

    to suitably prepared primed surfaces. When these water

    Quality nishes

    enhance

    appearance and

    promote long

    service life.

    borne nishes are used for re-painting it should be

    remembered that old, embrittled alkyd or oil-based

    nishes will continue to embrittle with time and will

    eventually fail irrespective of the type of re-paint top

    coat used.

    Exterior water borne nishes are generally based on

    emulsion resins, the most popular and proven type

    being pure or 100% acrylic emulsions.

    Semi-Gloss/SatinSatin and semi-gloss nishes, for both solvent and

    water borne coatings, are similar in many respects to

    their full gloss counterparts but have had the gloss level

    reduced by the addition of attening agents.

    While quite suitable for exterior use, when

    recommended by the manufacturer, they tend to chalk

    earlier but do not highlight minor imperfections in

    the substrate as readily as gloss nishes. They are

    frequently available in the same extensive colour range

    as gloss nishes but have an increased number of

    colours that are suitable only for interior use.

    Solvent borne semi-gloss nishes for exterior use are

    alkyd based, while water borne coatings are mainly

    100% acrylic based, to give maximum exterior

    durability.

    Flat/Matt and Low Sheen

    In this category the use of solvent borne nishes for

    exterior use is negligible and water borne nishes

    are almost universally preferred. The use of 100%

    acrylic emulsions gives very good exterior exposure

    performance over primed timber substrates where lm

    exibility on ageing is important.The term low sheen is generally used to indicate a

    nish with a slightly higher sheen than either at

    or matt. It is more washable and still masks minor

    imperfections in the substrate more satisfactorily than

    gloss or semi-gloss nishes.

    Natural Finishes

    Semi-transparent and opaque or solid stains have been

    developed to provide a more natural timber-look to a

    surface and to simplify maintenance procedures.

    The natural nishes are designed primarily for use on

    sawn timbers. Normally a one or two coat applicationof stain does not result in the thicker coating achieved

    with conventional paint systems, and therefore there

    is little risk of breakdown of the lm by cracking and

    aking. These stain nishes weather away by natural

    erosion, allowing subsequent re-coating with a minium

    of preparation.

    A simple brush and wash down to remove dirt and

    chalking pigment is all that is required. The periods

    between re-coating are generally shorter than with

    conventual systems, but tend to increase with

    subsequent re-paints.

    The build-up or thickness of lm of these natural

    nishes will vary from product to product. At the

    extremes it includes coatings which penetrate into

    the timber leaving little or no lm on the surface, to

    others which resemble top coat paints. These have

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    NAFITimber

    ManualDataleFM1-

    Exteri

    orFinishesforTimber

    10

    Care must be taken in using products of this type to

    avoid lap marks, as these clear exterior nishes dry more

    rapidly than an oil stain and a double application at the

    lap gives rise to bands of deeper colour.

    Application of Coating

    The most common application technique for exterior

    timber nishing is by brush, which although morelabour intensive, is specically recommended for initial

    priming coats as it gives better wetting of the surface

    and penetration into grain, joints and corners. Roller

    application or airless spray may be used for subsequent

    exterior coats and is more suited to large areas.

    The effectiveness of an exterior paint system is

    dependent on building a lm of adequate thickness and

    overspreading of coatings is a false economy. At all

    times the manufacturers recommendations, with regard

    to thinning, surface preparation, number and sequence of

    coats, maximum spreading rates, weather conditions and

    temperature at times of application and drying, and timeinterval between coats, should be adhered to.

    Apart from minimum times between coats, it is

    advisable that no longer than 2 weeks should elapse

    between coats without attention being paid to cleaning

    the surface to remove any grime or dust that may have

    built-up. Conditions at the site will obviously inuence

    the extent of this contamination. If the site is subject to

    salt contamination from prevailing winds off the sea,

    then washing down daily may be necessary.

    The application of stains, particularly semi-transparent

    types, should be in the direction of the wood grain

    and completed in one operation if lap marks are to beavoided.

    Factory priming or treatment with water repellent-

    containing preservative of some seasoned timber

    components, such as window assemblies, is designed to

    give them protection from weather and contamination

    during transport, site storage and an initial period after

    installation. But, as already mentioned, such factory pre-

    primed timber should not be considered as equivalent to

    properly primed timber.

    Joinery, cladding and other timber building materials

    delivered to the site unpainted should be primed all

    around as soon as practicable. Any cutting or handlingthat removes primer or leaves uncoated end-grain should

    be primed before nal xing.

    Likewise any primed work that has been allowed to

    deteriorate through exposure, should be cleaned down

    and re-primed.

    Exterior hardboard wall claddings are usually supplied

    factory primed. Some unprimed boards do not require

    priming when timber coloured, 100% acrylic nishes

    are used. The unprimed and primed boards should

    normally be paint nished within 60 days after xing, as

    recommended by the manufacturer.

    It is important that any surface to be painted, whether

    previously painted or not, should be suitably prepared to

    receive paint.

    Painting should not be undertaken during extremely hot

    weather or when temperatures fall below 10 degrees

    Celsius. Foggy, misty or dusty weather should also

    be avoided. Work for the day should be stopped early

    enough to allow for the lm to dry sufciently before

    adverse conditions develop.

    Maintenance

    Because of the wide variation in the severity of exposure

    conditions, the variability of timber substrates, thecare taken in the initial priming and nishing, and the

    number of systems applied over a period of years, it is

    impossible to predict the service life of a paint system

    before complete re-coating is necessary. Although on

    average, a period of 5 to 10 years may be expected, it

    is more likely that specic areas, such as those adjacent

    to timber end-grain, window sills facing north, or sharp

    arrised sections, may need remedial coating before this

    time. In order to minimise such action, it is advisable to

    give those areas an additional coat, either primer or top

    coat at the time of initial painting.

    The opposite situation arises on areas which are wellprotected, such as under wide eaves or verandas,

    which may not need re-coating on each repaint

    occasion, particularly as too great a build-up of paint

    will ultimately require major maintenance involving

    complete stripping before coating again.

    While excessive paint lm build up is to be discouraged,

    it is nevertheless most important to re-coat before

    general deterioration occurs.

    The presence of mould and mildew requires removal

    with fungicidal washes, and rinsing and drying before

    subsequent coats are applied. If this is not done, mould

    can spread between coats of paint and result in lifting ofthe paint lm and discolouration.

    Stains require more frequent maintenance but this is

    generally much easier to carry out both in terms of

    surface preparation and in coating application.

    Reference should also be made toDatale P4 -Timber-

    design for durability.

    The presence of mould and mildew requires removal

    with fungicidal washes, and rinsing and drying before

    subsequent coats are applied. If this is not done, mould

    can spread between coats of paint and result in lifting of

    the paint lm and discolouration.

    Stains require more frequent maintenance but this is

    generally much easier to carry out both in terms of

    surface preparation and in coating application.

    Reference should also be made toDatale P4 -Timber-

    design for durability.

    White

    opaque nish

    protectstimber and

    contrasts with

    dark nished

    sun-protected

    walls.

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