Protect from Sun
Most weathering of timber is caused by two mechanisms:
• Wetting and drying cycles that produce cracks
• Ultra-violet rays break down lignin, and cause degradation of wood fibre.
Detailing to avoid damage due to weathering can be addressed by two strategies:
• Shading or shielding - which incorporates the techniques outlined for protection against water, and includes the use of vegetation, overhangs, pergolas, screens, fences.
• Sealing with good quality paint.
Clear finishes reflect the beauty of the timber underneath, but do not generally provide the timber with UV protection. Opaque finishes last much longer and provide better protection.
Colour of finish is very important. Dark coloured surfaces absorb much more heat than light coloured surfaces. The heat causes accelerated degradation of the timber, and the paint system itself. Light coloured paint protection works MUCH better.
Maintenance of the paint systems is also very important. Once the paint has started to break down or crack, it allows water in, and traps it in the wood. It can do much more harm than good.
Some General Principles:
• Use white or light coloured opaque paints: pale colours absorb significantly less heat than dark colours and give reduced temperature-induced shrinkage, lower temperatures and less cracking.
• Shield surfaces with verandahs and roof overhangsthat prevent direct sunlight from falling on painted surfaces to enhance the durability of paint systems.
• Use acrylics paint systems: Unseasoned or treated timber is best protected by acrylic paint systems, as their greater porosity will allow moisture to move through the paint rather than blister it. Acrylic paints are also more flexible than most oil-based systems and can accommodate limited shrinkage better. For seasoned timber, it is a balance between the greater flexibility of acrylic paint systems and the better protective seal of oil-based paints. Oil-based paints allow less moisture movement and form a better protective seal around the timber. They should be used only where there is confidence that the timber will not shrink or swell in service..
• Match primers and undercoats to top coat: Any paint system must use good quality and well matched primers and undercoats to provide a good bond between timber and paint. Primers must be matched to the timber (including treatments) and to the paint system required for the top coats.
• Checks and open defects should be filled with an appropriate wood filler prior to the application of the primer coats. Paints are not able to fill gaps greater than 0.1 mm across, so if it is a check or gap that can be seen, then it should be filled with an appropriate filler. Different fillers are used for internal timber and for external timber.
• Sawn surfaces provide a better ‘key’ for stains and water repellents than dressed surfaces, particularly for denser species. The bond between paint and timber is a mechanical bond as the paint locks into the surface of the timber. Penetration of the paint into the timber is generally limited for higher density timbers with very thick cell walls and small pores. A sawn surface gives roughness that enhances the adhesion between the timber and its paint.
• Exterior grade stains and water repellents have a much shorter life than opaque paint treatments. This is particularly the case for coated surfaces exposed to sunlight. (Again verandahs and roof overhangs can be used to advantage to provide protection for stained surfaces). Clear finishes will require replacement at much closer intervals than equivalent opaque finishes. There are two reasons for this effect:
Clear finishes do not contain solid particles that reflect the energy of the sun and therefore oxidise sooner than opaque finishes that reflect much of the sun’s energy.
Clear finishes allow some of the energy to penetrate to the wood surface which can cause some deterioration of the interface between the paint and the wood. Not only is the wood sustaining damage, but the finish may separate from the wood prematurely.
• Coatings on rounded edges perform better. Paints tend to pull away from sharp corners when wet due to surface tension in the wet paint, which leaves a zone of thin coating at sharp corners. Rounded or arrissed edges retain paint better.