Effects of Wood on Durability of Finishes

Deterioration of wood is caused by physical (swelling and shrinking) and chemical (sunlight, air, and water) weathering and decay (fungi). Weathering wears away the surface at a rate of about 1/4 inch in a century. Decay occurs in damp areas and may be inhibited by using hardwood of a durable species or preservative-impregnated wood. Since wood is far more durable than paint, paint is primarily decorative rather than protective. Wood properties such as density, texture, extractives, and defects affect coating. Among softwoods, summerwood distribution is critical, since it releases paint more rapidly than springwood. Width of bands, determined by growth rate, density, and type of cut should be narrow to prevent paint loosening. Hardwood paintability is determined primarily by pore size, because weathering begins where pores are not filled. Extractives such as oils, resins, and watersolubles have less effect, though resins affect zinc oxide-containing paints, and poorly-dried lumber may result in dislodged or discolored coatings caused by exudations. Wood should be dried to moisture content midway between the extremes in service (about 12 percent in the greater U.S.). Knotty wood should be pre-varnished, or better, naturally finished, to avoid discoloration or dulling of paint. The Forest Products Lab. has classified species based on high quality lumber according to paintability. Generally, softwood cedars, baldcypress, and redwood work best (with pines affecting zinc oxide paints in all cases), followed by mixed hardwoods and softwoods. Large-pore hardwoods – ash, chestnut, elm, hickory, oak, and walnut are least suitable.

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