The use of semi-transparent exterior wood stains has grown remarkably since their development during the 1960s. This can be partly attributed to their offering a break with tradition at a time when it was being recognised that changes in the quality of timber and how it was used demanded different methods of exterior wood finishing. Information is already available on the characteristics and uses of exterior wood stains. This paper presents the current opinion on their performance and limitations, based on continuing research and considerable site experience. Until now there has been greater use of the so-called lowbuild stains which deposit little film onto the timber and so come closest to the ideal natural finish. They have a high moisture permeability, which has the benefit of reducing the level of any decay hazard but can lead to wide fluctuations in moisture content in service and to consequent problems of surface splitting and dimensional movement. In order to limit movement it is necessary to maintain the water repellency of the stain by periodic retreatment which, on exposed aspects, is likely to be necessary at intervals of about three years. High-build stains form a surface film about half the thickness of a conventional paint film and they possess a moisture permeability between that of paint and typical lowbuild stains. They exert more control on wood moisture content, reducing the problems of dimensional movement and surface splitting, and should not require such frequent maintenance as the low-build types. However, high-build stains are more prone to localised failure by flaking and this tendency will increase after a number of maintenance coats, which will also tend to obscure the natural features of the wood. Thus it is possible that the improved protection provided by high-build stains may be offset by losses in appearance and ease of maintenance, but there is at present insufficient experience of them in Britain to establish the extent to which this is so. Some high-build stains are based on water-borne acrylic agglutinants and have very good durability although when used as the sole treatment they lack the freshness of the solvent-borne materials and the thermoplastic nature of the film creates problems on contacting surfaces. Interest continues to grow in the opaque stains, or moisture-regulating paints. These materials share with stains the characteristics of being applied coat on coat instead of by the normal primer/undercoat/finish procedure, and of weathering by erosion. They have the appearance of satin finish paint but should possess improved flexibility and higher moisture permeability, and be easier to maintain. There has also been a revival in interest in the Swedish Royal Process which effectively combines wood preservation and decoration. The timber is vacuum-pressure impregnated with a water-borne preservative and is then subjected to a pressure treatment with a heated mixture of oil and pigments to confer colour and weather resistance to an outer zone of the wood. Experience suggests that the maintenance requirements of Royal-treated wood are much reduced.

Download document (247 kb)
free for the members of IRG. Available if purchased.

Purchase this document