There is a wide range of paints and stain types with selection determined by the degree of protection required and the appearance sought. Table 3 summarises the various finishes.
All paints and stains provide a coating on the surface and to some extent fill voids in the microscopic surface structure. Film forming (paints) and penetrating finishes (stains) vary in thickness and in the finished shape of the surface.
Film forming paints appear as a distinct layer and usually display a plane surface. Penetrating stains on the other hand follow the contours of the timber surface. Stains generally absorb into the grain and mirror the grain structure; paints can fill and bridge across the grain tending to smooth out and fill as shown in the diagram.
Due to the greater coating thickness, film forming finishes are more resistant to wear, and like penetrating finishes have the ability to accommodate differential movement of timber due to moisture variations, providing the correct film build is achieved.
The advantages of penetrating finishes include: a natural appearance; no peeling or blistering; suited to sawn textured surfaces; no trapped moisture in timber. They are also easily applied and renewed which is why they are best suited to rough sawn lumber. They can however be difficult to renew if the exposure has resulted in variations in the levels of weathering such as under overhangs. In this situation the application of new stain can lead to differential absorption into the surface and hence a patchy appearance. In this case moving to a film forming paint may be the best option.
Most coatings are impervious to water but none are completely impervious to water vapour. Pigmented paints at the correct film build are superior in this regard but can be subjected to some cracking or peeling. Protection is dependent on the paint’s continued adherence to the surface while subject to excessive UV and water uptake at the failed edge. Quick attention to this area will ensure the integrity of the paint film and the substrate is maintained.
Solvent borne paints are more resistant to water vapour than acrylic finishes but are not recommended as top coats. Where recommended by the manufacturer, a solvent borne primer and/or undercoat system may be used under water borne finishing systems to provide improved water vapour protection.
If a paint or stain is not applied on all surfaces of a piece of timber, differential moisture changes may cause distortion. Such problems are overcome by coating all the surfaces of the wood with a minimum of one coat as previously explained, reducing the speed at which moisture is absorbed and released, therefore creating a more dimensionally stable outcome.
Semi-transparent and opaque or solid stains have been developed to provide a more natural "timber-look" to a surface.
The natural finishes are designed primarily for use on sawn timbers. Normally a one or two coat application of stain does not result in the thicker coating achieved with conventional paint systems, and therefore there is little risk of breakdown of the film by cracking and flaking as most of the stain has soaked into the grain. Stain finishes weather away by natural erosion, allowing subsequent re-coating with a minimum of preparation providing that erosion is consistent over the whole surface. Where stain has not weathered such as under overhangs, re-staining may result in a patchy affect.
Stains generally last about 3-5 years north facing before re-staining is required; other sections may require no additional stain application. Before you decide on a natural look, make sure that in years to come you can get to those surfaces easily.
The build-up or thickness of film of these natural finishes will vary from product to product. At the extremes it includes coatings which penetrate into the timber leaving little or no film on the surface, to others which resemble top coat paints. If you’re not sure what look you want, do a sample board before you start.
Additional stain applied to end-grain will help prevent water absorption and hence improve performance by reducing the risk of premature failure through end-splitting or cracking.
Semi-transparent or Grain-enhancing stains
These are available in many colours and are formulated with relatively low pigmentation to enhance both the grain and texture of the timber. This relatively low pigmentation is sufficient to act as a U.V. screen. They have low film build properties and are easy to apply and maintain providing, as noted in natural finishes, all sections are subject to the same degree of erosion. Otherwise you can get a patchy result.
These stains are either solvent borne or acrylic. They contain fungicides to prevent mould growth on the stain itself and repel water, while still allowing some 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.
These are fairly heavily pigmented, low viscosity, low solids, paints. They allow the timber texture to show but obscure the grain pattern. Compared with the semi-transparent stains discussed above, the increased pigment, gives a longer service life. Re-painting, and surface preparation in particular, can be minimal compared to natural or semi-transparent stains because in most circumstances the erosion of the stain is more consistent or less obvious. A further coat is less likely to look patchy.
Water-borne Opaque Stains are also referred to as acrylic or solid stains and many are based on the 100% acrylic emulsions, although other co polymer 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 finishes are not generally recommended for timber elements that are fully exposed to the weather. Some good quality clear finishes incorporating U.V. absorbers are available. If a short service life is acceptable, a clear finish can be used successfully where attention to surface preparation and the number and types of coats are followed in detail.
Removal of sharp edges and filling and stopping of grain irregularities are essential, as is application under optimum conditions of both timber and atmospheric moisture content. Sealing of the end-grain to prevent moisture ingress is essential for these finishes to perform satisfactorily.
Maintaining a clear finish in good condition does require an on-going commitment to regular inspections and touch-up or re-coating of any areas where initial breakdown or discolouration is apparent.
Exteriors clears have been improved by the addition of UV absorbers. Most failure of the coating happens at the interface between the coating and the wood causing delaminating of the coating.
Water borne clears, based on specific pure acrylic emulsions, are showing considerable promise of extended durability but are not yet comparable to conventional paint systems. Inclusion of U.V. absorbers is essential. Some products also include very small quantities of pigments to obtain a natural timber colour.