timber finishes - lunch & learn

Timber Finishes

Project: Workshop & GarageLocation: Mt Albert, VictoriaBuilder: MCL Constructions

Timber Finishes ©FWPA 2011

Learn more about wood at UTAS

Centre for Sustainable Architecture with WoodGraduate Certificate in Timber (Processing & Building)• 4 units, part time, onlineAreas covered include:• Wood science• Design for durability and service for life• Timber as a renewable resource• Sustainable design and construction• Engineered wood products• International technologies and developments• Plus, selected topics of individual interestMore information: Associate Professor Greg Nolan (03) 6324 4478 or [email protected]


Learning Objectives

• After this presentation you should be able to:– Describe the basic types of finishes for timber and

wood products.– Understand the factors effecting finish

performance.– Specify finishes for timber and wood products.

• For architects - AACA Competency:– Documentation


Finish Purpose

• Finishes on timber have two purposes– to improve the durability

of the building– to add to the aesthetics

Project: The Letterbox HouseArchitect: McBride Charles RyanLocation: Blairgowrie, VictoriaImage: John Gollings


What is a Timber Finish

• Three types of timber finishes:– Stains • Contain pigments and are used to tint the timber

– Paints• Contain coloured pigments and sit on the wood to form

a protective coating

– Finishes • Paints without pigments that lay down a clear

protective coating


Why Finish Timber

• Unfinished, unprotected timber will inevitably weather as a result of gradual changes to its physical and chemical structure caused by temperature and moisture content variations.

• The weathering process leads to a slow breaking down and wearing away of surface fibres, changes in colour and roughening of the surface.

• The rate is generally slow, – Exterior – aprox. 6 mm per century

Mawson’s Hut, Antarctica


Finish Performance

• Performance depends on– choosing an appropriate finish– considering the use conditions– applying finishes correctly in sufficient amounts

Project: Outcrop House Architect: Peter Stutchbury Architecture Engineer: Simon MayLocation: North Beaches, NSWImage: Michael Nicholson


Selecting a Finish Goal

• Finish qualities– It must have the desired

appearance – It must protect the timber– It must be durable enough for

the intended application– It should be easily applied

Project: Herringbone House Architects: Alison Brooks Architects

Engineer: Price & MyersLocation: South London, United Kingdom

Image: Cristobal Palma


Types of Finishes

• Evaporative• Reactive• Coalescing

Project: Permanent CampingArchitect: Casey Brown Architecture

Location: Mudgee, NSW


Evaporative Finishes

• Evaporative finishes dry as their solvents disperse into the air

• They will always redissolve into the solvent originally used to thin them, making them easier to repair, but also less durable

• Common solvents are alcohol, acetone and nitro-cellulose lacquer thinners



• All finishes contain additives to control colour, drying time, sanding, etc.

• These additives are resins (solids) dissolved in a solvent. As the solvent dries the resin forms a film

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• Adding a new coat of evaporative finish to previous coats will dissolve the previous layer, allowing the new resin to bond with the previous coat

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• Types: Nitro-cellulose lacquers and shellac • Cellulose polishes and thinners and lacquer

and lacquer thinners are the in the same family

• Wax is an evaporative finish because it is dissolved in turpentine or petroleum distillates to make the familiar soft paste. After these distillates evaporate, the wax remains.



Product Ease of application Repairability

Health and

safety Water

resistance Chemical resistance

Scratch resistance

Wax Excellent Excellent Good Poor Fair Poor

Shellac Good Excellent Excellent Fair Poor Fair

Nitrocellulose lacquer Fair Excellent Poor Good Good Fair

Most water-based

finishesGood Poor Fair Good Good Good


Reactive Finishes

• Reactive finishes are those where the resin undergoes a chemical change as the solvent evaporates

• The resins crosslink to form new large molecules• Use solvents such as white spirits and naphtha

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Reactive Finishes

• Oil varnishes and linseed oil are reactive finishes, as they change chemically as they cure

• At cure, the solvent/thinner evaporates and the resins cluster tightly together, a chemical reaction then occurs, which causes the resins to cross link in a different chemical format


Reactive Finishes

• Scuff sanding is necessary between layers of cured finish so that subsequent applied layers have something to grip to.

• The solvent won't re-dissolve the cured film, e.g., white spirits do not soften cured oil based varnish.

RR R R RRRSmooth Surface

R R R

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Reactive Finishes

• The oil based varnishes dry from the top down by reacting with oxygen.

• The catalysed lacquers dry from the bottom up (which is like evaporative finishes) and the solvents migrate upwards to the film surface and then out leaving behind molecules that then crosslink.

• Tung oil and linseed oil are reactive finishes that cure by reacting with oxygen, but do not really form film finishes when cured.

• Water based finishes generally fall into the coalescing category.


Reactive Finishes

Product Ease of application

Repairability Health and safety

Water resistance

Chemical resistance

Scratch resistance

Linseed oil Excellent Excellent Excellent Poor Fair Poor

Tung oil Excellent Excellent Excellent Fair Fair Poor

Oil-based varnish (alkyd resins)

Good Poor Poor Excellent Good Good

Oil-based polyurethane

Good Poor Poor Excellent Excellent Excellent

Catalyzed lacquer (and conversion varnish)

Poor Poor Poor Excellent Excellent Excellent


Coalescing Finishes

• Generally in water based finishes, droplets of resin are suspended in an emulsion of water and slow drying alcohols – glycol ether

• As water evaporates, the glycol ether softens the resins which then fuse together to form a dry film


How do Finishes Settle on Timber

• Penetrating Finish• Surface Film Finish• Penetrating Film Finish

Project: Sunset CabinArchitect: Taylor Smyth ArchitectsLocation: Lake Simcoe, CanadaImage: Ben Rahn


Penetrating Finishes

• Finishes absorbed into the pores of the wood

• Generally oil finishes – Tung– Linseed– Danish oil

• Disadvantage - offer little protection from water, chemicals, and wear

• Advantage – easy to apply and repair damage


Surface Film Finishes

• Finishes that do not penetrate into the wood but dry to a film on the surface

• Wax and water based finishes• Advantages – resist moisture, stains, and wear• Disadvantages – lack colour of oil finishes ,

repairing requires more work, adhesion of one coat to the next may be difficult


Film Forming v Penetrating Finish

• Film Forming– Film forming finishes

appear as a distinct layer and usually display a plane surface.

– Due to the greater coating thickness, film forming finishes are more resistant to wear


Film Forming v Penetrating Finish

• All finishes provide a coating on the surface and to some extent, fill voids in the microscopic surface structure

• Film forming and penetrating finishes vary in thickness and in the finished shape of the surface

Project: Surry Hills Community CentreArchitect: Francis Jones Morehen ThorpLocation: Surry Hill, NSW


Penetrating Film Finishes

• Finishes that form on the surface as well as penetrate into wood pores

• Examples: nitrocellulose lacquers, oil based varnish, polyurethanes

• Advantages – film forming properties make them more ware resistant, and penetrating qualities give better finish qualities

FACTORS AFFECTING FINISH PERFORMANCE

Project: Seaford Surf Life Saving ClubArchitect: Robert Simeoni ArchitectsLocation: Seaford, Victoria


Factors Affecting Performance

Factor Effect on PerformanceSubstrate More later

EnvironmentalConditions

Dry temperate conditions favour very good performance compared with tropical, marine and severe industrialenvironments which provide relatively severe exposure conditions

Pre-treatment Use of preservative impregnation or water repellent preservative treatment before painting, together with puttiesand sealants where appropriate, will improve service life

MaintenanceSchedule

Re-coating before major paint film breakdown occurs will improve durability and reduce costs


Factors Affecting Performance

Factor Effect on PerformanceApplicationProcedure

Adequate film build and attention to coating end-grain results in improved performance

BuildingDesign

Protection from elements using roof overhangs, verandas, pergolas, etc., and avoiding water traps will improve performance

System choice Correctly choosing a system for each job and using one manufacturer’s products in each system will contribute to the long term performance of the finish


Wood Science

HeartwoodSapwood

Quarter sawn Back sawn


Wood Science

Earlywood

Late wood

Douglas fir tree rings


Substrate – Wood Itself

SubstrateVariable

Effect on Performance

Species Differs between and within species – more later

Density The heavier, denser timbers are slower at moisture uptake than lighter species

Dressed timber Better performance with conventional paint systems

Sawn timber Better performance with solid and natural stains

Timber edges Sharp edges create stress in paint films, causing failure. Arrised or rounded edges permit film movement with minimal stress

Back-sawntimber versusQuarter-sawntimber

Quarter-sawn softwood boards generally have better dimensional stability than back-sawn, and therefore better finish holding properties. For hardwood the difference is minimal



SubstrateVariable


Weathered versusUnweathered timber

Surface deterioration of timber fibres due to weathering can result in poor paint adhesion unless corrective sanding is carried out. Stains may perform better than conventional finishes if sanding is omitted

Extractives May cause topcoat discolouration or blistering unless surface extractives are removed with a solvent wash prior to priming.Some coatings require a long exposure to elements before application

Heartwoodversus Sapwood

Heartwood has a higher staining potential than sapwood but is ideally suited to painting with exterior timber stains



SubstrateVariable


Earlywoodversus Latewood

Cracking on latewood is more likely to occur as the solvent borne paint becomes brittle after extended exposure

Earlywoodversus Latewood

Coatings last longer on wood with narrow latewood bands due to density differences between the earlywood and the denser latewood

Smooth texture versus coarse Texture

Smooth texture surfaces have better paint holding potential than coarse texture ones. Using stains on the latter gives good performance



SubstrateVariable


Gum pockets Can lead to resin exudation and staining unless pre-treated and sealed. Aromatic oils can lead to drying retardation and staining if surface oils are not removed

Knots Possible premature cracking, staining and resin exudation can occur unless treated with knotting varnish or manufacturers recommend treatment

Bark Can lead to premature failure of all film-forming finishes if not removed

Insect Damage Premature failure may occur if not treated or filled before finishing



SubstrateVariable


MoistureContent

Normal range 10-15% - higher levels are detrimental, particularly for solvent borne finishes where moisture vapour permeability is low

MoistureContent

Coatings last longer on dimensionally stable species such as western red cedar, as these shrink and swell less than other species and therefore put less stress on the coating bond

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


Substrate

• Summary – The density, moisture content, absorbency,

flexibility and the nature of its extractives may vary considerably

– The finishing system must be sufficiently flexible to cater for movement


Moisture

• Moisture – Use wide eave overhangs, verandas,

pergolas etc.– Use timber profiles with rounded

arrises– Fit drip caps over doors and windows– Avoid or minimise joins in horizontal

sidings likely to allow moisture ingress

– Use sacrificial timber that can be easily replace or maintained instead of critical element

Project: Dunsborough ResidenceArchitect: Wrightfeldhusen

Location: Dunsborough, WAImage: Robert Frith


Moisture

• Moisture – Provide adequate ventilation of

roof space and subfloor space– Use the recommended nail size

and pattern for various types of timber cladding

– Selectively use boards that show knots or the like. Use them in the least exposed areas


Surface Preparation

• The purpose of preparing a substrate is to ensure that it will accept and retain the finish with minimum interference from surface contaminants or from surface deterioration of the substrate itself

• Factors affecting this are: – the profile of the surface and its porosity– adhesion of the paint system and its ultimate durability.

• A 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 film on the surface peaks which will deteriorate more rapidly. Some surfaces are, however, ideally suited to opaque or semi-transparent stains


• If timber has been exposed to the elements for a considerable time and suffered the weather’s oxidative effects, it should 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 should be scraped clean, primed and filled with putty compatible with the paint system

• Emphasis must be placed on preventing moisture ingress, particularly where end-grain is concerned. Adequate sealing, with sealants, water repellents or primers is essential

Surface Preparation


Extractives

• Extractives vary between species. Some extractives are water soluble and abundant in woods, such as merbau, blackbutt, tallowwoods and other dark coloured wood

• Extractives can discolour the finish, as water and high relative humidity can transport them to the coating surface


Resins

• Resin can be found in most softwoods, e.g. pine, spruce, or fir

• Resin is a mixture of rosin and turpentine, and can migrate through the wood

• Typically, kiln drying wood helps drive off the turpentine, making the rosin less mobile


Other Considerations

• Timber with an excessively high moisture or resin/aromatic oil content should undergo further seasoning to rectify the conditions before painting

• 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 starting work, while still allowing time for the surface to dry


Other Considerations

• Timber with excessively high moisture or resin/aromatic oil content should undergo further seasoning to rectify the conditions before painting

• Some coating manufacturers suggest that the timber weathers before applying the finish – Refer to the coating manufacturer for guidance


Permeability

• Most coatings are impervious to water but none are completely impervious to water vapour

• Surface films are superior in this regard but after initial cracking or peeling, protection is dependent on the finish that has penetrated into the wood

• Penetrating finishes may therefore be more effective


Permeability

• Solvent borne finishes are more resistant to water vapour than water borne finishes, so where a high level of protection 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 finishing system to provide improved water vapour protection


Semi-Transparent Stains

• These are available in typical timber colours and are formulated with relatively low pigmentation to enhance both the grain and texture of the timber

• Low pigmentation act as a U.V. screen to some extent

• The pigment also remains as a colouring for the timber surface after the natural timber colours have faded

• They have low build-up properties and are easy to apply and maintain


Semi-Transparent Stains

• These stains are usually solvent borne. They contain fungicides to prevent mould growth on the stain itself and repel water, while still allowing moisture vapour permeability

• Pre-treatment of the timber product with a water repellent-containing preservative or pressure preservative treatments, will extend the expected life of the system. However, if used alone, such pre-treatments will not prevent deterioration by weathering


Opaque Stains, Solvent Borne

• These are fairly heavily pigmented, low viscosity, low solids, flat paints. They allow the timber texture to show but obscure the grain pattern.

• The increased pigment, compared with semi-transparent stains, gives a longer service life

• Re-painting, and surface preparation in particular, is minimal compared with conventional paint systems


Opaque Stains, Water Borne

• These are also referred to as acrylic or solid stains. Many are based on 100% acrylic emulsions, although other polymeric emulsion binders can be used

• They are now used extensively as an alternative to oil-based solid stains and generally give superior performance, particularly over knotty timbers


Clear Exterior Finishes

• Clear finishes are generally not recommended for timber elements that are fully exposed to the weather

• Some good quality clear finishes incorporating U.V. absorbers are available but have not as yet been proven to have a long service life in Australian conditions



• If a short service life is acceptable, a clear finish can be used successfully where attention to surface preparation is followed in detail:– remove sharp edges and fill and stop grain irregularities– apply only under optimum conditions of both timber and atmospheric

moisture content– sealing the end-grain and all round to prevent moisture ingress is very

important for the satisfactory performance of these finishes

• Maintaining a clear finish in good condition does require an ongoing commitment to regular inspections and touch-up or recoating of any areas showing initial breakdown or discolouration



• Solvent borne exterior clears, usually in alkyd or alkyd/ urethane vehicles, have been improved by the inclusion of U.V. absorbers

• Water borne clears, based on specific pure/acrylic emulsions, are showing considerable promise of extended durability comparable to conventional paint systems, provided regular maintenance is carried out. Inclusion of U.V. absorbers is essential

• Some products also include small quantities of pigments to obtain a neutral timber colour


Paints

• Pre-Primed TimbersSome timbers and wood based building boards are supplied pre-primed and provided they are not exposed to the elements for extended periods, may be finished with undercoat and an enamel paint or two coats of an acrylic paint without further priming (or as recommended by the manufacturer)


Paints

• UndercoatThese are the bridging coats between the primer or previous paint surface and the finishing coat. They have 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


Paints

• Undercoats– Similar to primers, they can be solvent or water

borne– Solvent borne undercoats are better for adhesion

if applied to slightly chalky or powdery surfaces– Water borne undercoats have better colour and

non- yellowing qualities. They are faster drying with much shorter times before overcoating

– They may be overcoated with either water or solvent borne coatings


Paints

• Differences in paint performance over earlywood and latewood can occur particularly on softwoods where latewood is generally harder, smoother and darker than earlywood

• Paints with a tendency to become brittle with age (i.e., those that are solvent borne) may ultimately crack and peel away from the harder surface despite having good initial adhesion

• Uniformly fine textured timbers have better paint holding characteristics than coarse textured species


Paint Colour

• Research has demonstrated that externally, lightly coloured paints provide much better service life for both the timber and paint

• Dark coloured surfaces heat up significantly more than light coloured surfaces and this will cause timber to dry to a lower moisture content than it otherwise would. The subsequent greater shrinkage and movement may lead to checking, splitting and short term cupping

• Higher temperatures also promote resin bleed in some boards of species that can be prone to it

• As a rule of thumb, the Light Reflective Value (LRV) of the paint should be greater than 30%. Light reflective paints should be used where possible.


Paint

• Quarter-sawn boards, tend to hold paint better• In hardwoods the difference is comparatively

small compared to that of softwoods• As quarter-sawn boards tend to be more

dimensionally stable, they are better under variable moisture conditions and less likely to show surface checking to the detriment of the paint system

• Most parcels of timber are supplied as a mix of back-sawn and quarter-sawn pieces so this maybe hard to arrange


Weathering

• Sunlight quickly degrades the ability of a wood surface to bond with a coating

• Research has shown a big difference in paint performance on weathered versus unweathered wood

• Paint on boards with no exposure to weather prior to painting lasts many times longer than boards weathered prior to painting

• For maximum coating life, sand the surface if the wood has been exposed to any sunlight, particularly if for more than two weeks


Exterior Timber

Importance of sanding• Sanding (100 grit) can double the life of a

coating, for both weathered and freshly planed wood. This is because sanding removes damaged surface fibres and changes the surface chemistry to improve bonding of the coating


Exterior Timber

• Plywood – Plywood is characterised by the small cracks (face checks) on the surface

that are caused by the lathe when the veneer is cut from the log – As plywood goes through moisture cycling outdoors, these cracks tend to

get larger and stress the coating bond– Generally a good stain can effectively protect plywood. Since checking in

stained plywood usually occurs during the first six months of outdoor exposure, best coating results can be obtained by applying a first coat and allowing any checking to occur, then allow six months or so and then apply a second coat

– Paints service life can be reduced unless efforts are made to reduce moisture uptake and also to use flexible products to accommodate dimensional changes of the wood. Roughening the surface is also important


Exterior Timber

• Finger-jointed products: – Coatings may perform differently on different

parts of these products, as they may not be uniform in grain orientation or in heartwood versus sapwood content.

– Roughen the surface to extend the life of the coating and minimise these differences

– Apply primer and paint all sides if possible to minimize moisture absorption


Exterior Timber

• Because exterior wood shrinks and swells with moisture changes, the coating needs to be flexible. Flexibility varies by product – some products may be clearly identified as suitably flexible for wood’s dimensional changes

• Water borne coatings are generally more flexible than alkyds

• Coatings containing urethanes tend to be more flexible than coatings containing acrylics


Specifications

• The following is a check list of items which should be included in the specification of timber finishes– selected finish - brand, product range, colour– finish system - primer, undercoat, finish coats

(number) – substrate preparation– timber edges to be arrised or rounded– priming of joints– extra coats - sills, rails, etc.– maintenance


Maintenance

• As there is a wide variation in the severity of exposure conditions, the variability of timber substrates, the care taken in priming and finishing, 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

• It is more likely that specific areas, such as those adjacent to timber end-grain, window sills facing north, or sharp arrised sections, may need remedial coating before this time

• To minimise such action, it is advisable to give those areas an additional coat of either primer or top coat during initial painting.


Maintenance

• Areas that are well protected, such as under wide eaves or verandas, 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 recoating

• While excessive paint film build up is to be discouraged, it is important to re-coat before general deterioration occurs


Maintenance

• Stains require more frequent maintenance but this is generally much easier, both in terms of surface preparation and coating application

• 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 film and discolouration


Check List: Selecting a Finish

• To determine the best finish for a project, consider three factors: – appearance– application– durability

• How will the item be used? Will it be subjected to a lot of moisture, solvents, and dents?

• What do you want the wood to look like? Do you want an "in-the-wood" natural look or a thicker film finish that accentuates depth?


Specifications

• The following is a check list of some items which should be included in the specification of timber finishes:– selected finish - brand, product range, colour– finish system - primer, undercoat, finish coats (number) – substrate preparation– timber edges to be arrised or rounded– priming of joints– extra coats - sills, rails, etc.– maintenance


Summary

• There is no single timber finish or system that will work for all applications and timber species

• Good specification and attention to details and site inspections will enhance service life

• Manage the expectations of your client – e.g. furniture quality wood finish won’t last

outside


More Information

WoodSolutions Technical Design Guides


Learn more about wood at UTAS

Centre for Sustainable Architecture with WoodGraduate Certificate in Timber (Processing & Building)• 4 units, part time, onlineAreas covered include:• Wood science• Design for durability and service for life• Timber as a renewable resource• Sustainable design and construction• Engineered wood products• International technologies and developments• Plus, selected topics of individual interestMore information: Associate Professor Greg Nolan (03) 6324 4478 or [email protected]

timber finishes - lunch & learn

Self Improvement

basic types of finishes

new coat of evaporative

layers of cured finish

timber finishesproject

unprotected timber

finish performanceperformance

finish timberunfinished

finish purposefinishes