Log Preparation is a Key to Stain Performance
from SASHCO, INC. – LOG HOME BASICS
The following is a discussion of the difficulties involved in planning for and carrying out the finishing of log structures (or most exterior wood) with coatings. The greatest single challenge is to have properly prepared wood substrates available when the chosen coating is applied to the logs (or exterior wood). "Properly prepared wood," means that the surface is clean, sound, warm and dry. If a coating is applied to a log surface that meets only one, two or three of the four needed criteria, then there will be a strong probability of failure of the coating- much sooner and more catastrophically than would have otherwise been expected. Most, if not all, of what follows is known by well-informed, professional painters and contractors since this information has been disseminated for many years by professional paint-trade organizations, the USDA Forest Products Laboratory (Madison, WI) and other groups. What follows are not paint-trade organizations, the USDA Forest Products Laboratory (Madison, WI) and other groups. What follows is not rocket science, but, rather common sense. Painting professionals have historically found that over 75% of all coatings failures are due to poor substrate preparation, poor application methods, or both – seldom is it just the coating that fails.
A clean wood surface is probably the requirement that needs the least explanation since it is generally understood by most people that if a surface is dirty (with actual "dirt", grease, oil, pollen, wax, mold, mildew, peeling paint, mill glaze, etc.) There is little chance that a newly applied coating can "wet out", penetrate or establish good adhesion to such a surface. However, because the amount of work sometimes required to get a surface completely clean can be substantial, many times short-cuts are taken and a really clean surface is never achieved – with inevitably poor results. Even though some work is required (occasionally substantial work!); make sure the surface is clean.
Wood degrades very fast when exposed to UV light, as the USDA Forest Products Laboratory (among others) has shown in many studies over the years. Chemists from the Forest Products Laboratory concluded (for example) in the November/December 1987 issue of the Forest Products Journal (page 30-31): "Adhesion of both acrylic latex and alkyd oil primer paints to wood significantly reduced after the wood finishes in southern Wisconsin. We anticipate a greater effect in warmer, and, especially, sunnier climates."
The reason is that the coating may establish good adhesion to the fragments of the UV-damaged wood at the surface but those fragments themselves are poorly attached to the bulk of the wood substrate because of the UV degradation. So, if logs have been left to the weather for an extended period, they then need to be sanded, media-blasted or aggressively power washed (to remove damaged surface fibers of wood) before any coating is applied.
The surface of the wood should not be at any extreme temperature when a coating is applied, regardless of whether the coating is water-based or oil-based. The reasons are simple: 1) If a surface it too hot (such as with the upper curvature of a log in direct sunlight), then the coating (whether oil-or water-based) can dry far too fast to permit proper wetting, penetration and adhesion. 2) If a surface is too cold, then penetration of oil-based coatings can be reduced because of the contracted state of the wood and the increased viscosity of the coating, and water-based products can catastrophically fail because the water can freeze before allowing proper film formation and evaporation (among other damaging things!). Only apply coatings in consistently warm weather so that the temperature is not only warm when applied but also during cure; but, avoid extremely hot weather and especially avoid surfaces in intense, direct sunlight.
It is extremely important to apply coatings to dry wood. "Dry" has been defined by the USDA Forest Products Laboratory as being 20% or less moisture. As the USDA publication Finishing Wood Exteriors (Agriculture Handbook No. 647, 1986) says on page 7: "If the moisture content of the wood exceeds 20% when it is painted, blistering and peeling are likely". Use a wood moisture meter. Logs, of course pose an additional problem because of the horizontal checks that occur in the upper curvature of the exterior log surface which catch rain and snow and water and allow the moisture to soak into the wood behind whatever surface coating may have been previously applied. This fact of log home living underscores the need for caulking the larger checks and thoroughly brushing any coating into the surface to seal the smaller checks (versus just spraying – which can never allow the coating to really get into the checks of a log). If the logs are not dry (and often, because of their mass, they are not), then let them dry out before coating: and if drying means a required long exposure to the sun, then the logs will need to be sanded, media-blasted, or strongly power-washed before the coating can be applied to remove the inevitable unsound wood that develops from the UV exposure.