Clear exterior finishes: finding the balance between aesthetics and durability

JCT CoatingsTech, Sept, 2004 by Tom Daniel, Marc S. Hirsch, Ken McClelland, Alan S. Ross, R. Sam Williams

Moisture

Water also degrades wood. Whether in the form of liquid or vapor, absorption and desorption of water causes dimensional changes of the wood, which can lead to premature degradation of the finish. Water causes cracking, checking, and warping of the finish and can also lead to decay (rot) in wood. In general, it accelerates the weathering of wood exposed outdoors.

SHRINKING AND SWELLING: Wood shrinks as it loses moisture and swells as it gains moisture. More precisely, wood only changes dimension between an absolutely dry state (completely free of moisture) and its fiber saturation point (the point at which the cell walls of the wood fibers are completely saturated with moisture). This fiber saturation point typically occurs at about 30% moisture for most species of wood. At this point, all the wood's water is bound within the cell wall. As moisture content changes above fiber saturation, the cell cavities take on or lose unbound water but the wood cell walls do not change dimensionally. Below the fiber saturation point, however, the wood will change dimension with changing moisture content.

WATER AND WATER VAPOR EFFECTS: Shrinking and swelling of wood occurs whether the water is in the form of vapor or liquid. For example, wood swells during periods of high humidity and shrinks during periods of low humidity. If wood is exposed to 100% relative humidity (RH) for an extended period, the moisture content will eventually reach the fiber saturation point but it will not go beyond this point without the presence of liquid water. Wood exposed outdoors goes through a daily and seasonal change in moisture content as determined by the RH. It is almost never exposed to extended periods of 100% RH. Even in the hot humid areas of the south, the moisture content of wood stays well below 20% as long as the wood does not get wet. Water, however, can cause wood to quickly reach, and even go beyond, fiber saturation. The source of this water can be rain, dew, melting ice or snow, plumbing leaks, and condensation of water vapor as it moves in or out of a building. The dimensional changes caused by daily and seasonal changes in RH have little effect on the performance of wood finishes.

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Poor performance of wood finishes may occur when the moisture content of wood reaches or goes beyond fiber saturation. A large percentage of wood finish degradation (e.g., paint or stain defects, peeling, cracking) results from moisture changes in the wood and subsequent dimensional instability. Water also decreases paint bond strength, thus increasing the chance for peeling. The wetting-drying cycles of wood exposed outdoors can raise the grain of the wood. If the raised grain is severe, it can cause cracks in paint, particularly in oil-based paints.

For wood finished with clear penetrating finishes, dimensional changes caused by wetting-drying cycles may lead to raised grain, checking, and cracking of the wood. This roughened surface increases water uptake, thus accelerating the degradation process. On flat-grained lumber, the raised grain may appear as thin knife-like feathers along the earlywood-latewood interface.