The function of surface coatings (paints and stains) on timber is two-fold; to improve the durability of the building and to improve the aesthetics.
Unfinished, unprotected timber will inevitably weather as a result of gradual changes to its physical structure brought about by photo-chemical attack, abrasion, and 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 fibres, 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 local conditions.
In most applications timber needs protection from the elements of water, cold, heat and ultra-violet light to give it a longer life. Paints and stains also have decorative benefits.
The performance of exterior paints and stains depends on a wide range of factors. The local environment however, is a variable which needs special consideration and local experience can be beneficial. Advice should be sought from manufacturers of both the timber product and your preferred paint brand when investigating suitability for your particular region.
The selection and correct application of the appropriate paint or stain product is one of the most important components of the design and construction process. It’s essential that the manufacturer’s recommendations for both the wood and the paint or stains are followed in relation to preparation, product application and material maintenance.
References for this sub-section:
1. AS/NZS 2311 Guide to the painting of buildings: Standards New Zealand
2. Finishes for Exterior Timber. Trada (UK).
3. Surface Coatings volumes 1 and 2.
4. Oil and Colour Chemists Association of Australia.
5. Finishing Wood Exteriors: Selection, Application and Maintenance: US Department of Agriculture Handbook no.647.