Preservatives stains as exterior wood finishes

IRG/WP 389

D W French

For many years wood preservatives and paints have been used as the only treatment for exposed wood surfaces. Because of the inherent color of the preservatives, such as creosote, the wood surface was stained as well as protected from attack by micro-organisms. Paints protect surfaces from weathering, but recently, with an increased interest in maintaining the more natural appearance of exterior wood surfaces, painting is being replaced by staining. Stains have the advantages, in addition to preserving the natural wood appearance, that they do not peel, blister or crack, can be applied easily and renewed with little difficulty. The cost is less than that of paint, initially and over the life of the building. One of the major attractions for homes is that stains blend in with the environment. If the home owner desires a white exterior or bright colors, paint rather than a preservative stain must be used. Paints are more effective in protecting the wood from damage by sunlight and will avoid discoloration of the exterior surface resulting from the natural extractives present in wood. Special aluminium or stainless steel nails are not needed with paints, but are needed with stains. Preservative stains can be classified in various ways. Wood preservatives, such as creosote and pentachlorophenol, have been used for many years primarily for preventing deterioration caused by fungi, including fungus species which discolor and those that decay the wood. These compounds provided some degree of water repellency, but this was not a major objective in their use. Later, water repellents were added to the preservatives to reduce the rapid uptake of moisture and thus provide some dimensional stability to the product. More recently water solutions of inorganic salts have demonstrated that they provide protection to the wood, add color and are beneficial primary treatments if the wood surface is to be painted. Some of the water soluble formulations may provide some fire retardant properties as well. A common system of classification today includes three general types of natural finishes as follows: 1. water-repellent preservatives; 2. pigmented penetrating stains; 3. inorganic water soluble salts. It is possible to obtain a satisfactory exterior surface if no treatment is applied. Such an approach, however, is satisfactory only where the weather is not favorable for micro-organisms. If the fungi and other micro-organisms are allowed to develop, the coloring will be very nonuniform, the wood surface will deteriorate, and warping and the like will occur to excessive degrees. Thus in most environments one of the above systems must be used.


Conference: 77-09-26/30 Noordwijk aan Zee, The Netherlands

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