Timber Finishes - Lunch & Learn

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This presentation provides guidance on coatings used for interior and exterior wood surfaces to enhance appearance and/or the natural durability. Selecting the appropriate coating product is essential …

This presentation provides guidance on coatings used for interior and exterior wood surfaces to enhance appearance and/or the natural durability. Selecting the appropriate coating product is essential to maintain visual appeal and ongoing performance of the wood.

Every wood species has characteristics that can affect the application of products. It is important to know about the wood being coated and how to reference manufacturers' product literature for specific recommendations.

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  • 1. Timber Finishes
    Project: Workshop & Garage
    Location: Mt Albert, Victoria
    Builder: MCL Constructions
  • 2. Learn more about wood at UTAS
    Centre for Sustainable Architecture with Wood
    Graduate Certificate in Timber (Processing & Building)
    4 units, part time, online
    Areas 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 interest
    More information: Associate Professor Greg Nolan
    (03) 6324 4478 or enquiries@arch.utas.edu.auwww.csaw.utas.edu.au
  • 3. 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
  • 4. Finish Purpose
    Finishes on timber have two purposes
    to improve the durability of the building
    to add to the aesthetics
    Project: The Letterbox House
    Architect: McBride Charles Ryan
    Location: Blairgowrie, Victoria
    Image: John Gollings
  • 5. 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
  • 6. 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
  • 7. 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 May
    Location: North Beaches, NSW
    Image:Michael Nicholson
  • 8. 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 & Myers
    Location: South London, United KingdomImage: Cristobal Palma
  • 9. Types of Finishes
    Evaporative
    Reactive
    Coalescing
    Project: Permanent Camping
    Architect: Casey Brown Architecture
    Location: Mudgee, NSW
  • 10. 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
  • 11. Evaporative Finishes
    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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  • 12. Evaporative Finishes
    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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  • 13. Evaporative Finishes
    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.
  • 14. Evaporative Finishes
  • 15. 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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  • 16. 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
  • 17. 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.
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    Smooth Surface
  • 18. 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.
  • 19. Reactive Finishes
  • 20. 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
  • 21. How do Finishes Settle on Timber
    Penetrating Finish
    Surface Film Finish
    Penetrating Film Finish
    Project: Sunset Cabin
    Architect: Taylor Smyth Architects
    Location: Lake Simcoe, Canada
    Image: Ben Rahn
  • 22. 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
  • 23. 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
  • 24. 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
  • 25. 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 Centre
    Architect: Francis Jones Morehen Thorp
    Location: Surry Hill, NSW
  • 26. 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
  • 27. Factors Affecting Finish Performance
    Project: Seaford Surf Life Saving Club
    Architect: Robert Simeoni Architects
    Location:Seaford, Victoria
  • 28. Factors Affecting Performance
  • 29. Factors Affecting Performance
  • 30. Wood Science
    Quarter sawn
    Back sawn
    Heartwood
    Sapwood
  • 31. Wood Science
    Douglas fir tree rings
    Earlywood
    Late wood
  • 32. Substrate – Wood Itself
  • 33. Substrate – Wood Itself
  • 34. Substrate – Wood Itself
  • 35. Substrate – Wood Itself
  • 36. Substrate – Wood Itself
  • 37. 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
  • 38. 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 Residence
    Architect: Wrightfeldhusen
    Location:Dunsborough, WA
    Image: Robert Frith
  • 39. 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
  • 40. 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
  • 41. 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
  • 42. 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
  • 43. 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
  • 44. 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
  • 45. 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
  • 46. 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
  • 47. 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
  • 48. 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
  • 49. 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
  • 50. 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
  • 51. 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
  • 52. 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
  • 53. Clear Exterior Finishes
    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
  • 54. Clear Exterior Finishes
    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
  • 55. Paints
    Pre-Primed Timbers
    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 finished with undercoat and an enamel paint or two coats of an acrylic paint without further priming (or as recommended by the manufacturer)
  • 56. Paints
    Undercoat
    These 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
  • 57. 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
  • 58. 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
  • 59. 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.
  • 60. 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
  • 61. 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
  • 62. 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
  • 63. 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
  • 64. 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
  • 65. 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
  • 66. 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
  • 67. 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.
  • 68. 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
  • 69. 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
  • 70. 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?
  • 71. 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
  • 72. 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
  • 73. More Information
    WoodSolutionsTechnical Design Guides
  • 74. Learn more about wood at UTAS
    Centre for Sustainable Architecture with Wood
    Graduate Certificate in Timber (Processing & Building)
    4 units, part time, online
    Areas 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 interest
    More information: Associate Professor Greg Nolan
    (03) 6324 4478 or enquiries@arch.utas.edu.auwww.csaw.utas.edu.au