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

Consumers can easily be confused by the abundance of choices to make when selecting a clear wood finish. There are many types of clear finishes with different characteristics and product claims. This article is designed to help consumers sort out the different finishes and effectively choose which product would be best for their purpose. First, we cover the causes and mechanisms of wood degradation. It is important to know what we are protecting wood from and why. Then we describe in detail the different kinds of clear wood finishes and how they are designed to work. We also cover the attributes of wood as a building material, which explains why people are so interested in using it in the first place.

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INTRODUCTION

Wood has long been admired for its natural beauty, durability, and strength. Because of the increasing value of wood used for exterior applications, such as siding, decks, walkways, fences, gazebos, etc., homeowners want to protect their investment by applying a protective finish to the wood. Homeowners and professional painters have traditionally used protective finishes, such as paint, but these finishes hide the natural beauty of the wood. In many cases, the homeowner wants the natural beauty of the wood to show through the finish. This natural wood appearance, although desirable, is difficult to maintain. The harmful effects of nature (primarily sun and rain) degrade the wood surface, both aesthetically and functionally. In recent years, a number of clear wood finishes have been developed that protect wood while accentuating its natural beauty (Figure 1). Although the consumer can choose from a vast array of finishes that have a wide range of pigments

and hiding power, the focus of this paper is on those finishes that have little visible pigment. In some cases, the pigments are included but are finely ground so that the coating is transparent to visible light.

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Most natural clear wood finishes are designed to protect and beautify wood. Although this may sound simple, manufacturers' claims that products protect, preserve, seal, last longer, work better, and spread farther than competitors' products are often conflicting and can confuse both the homeowner and professional painter. In fact, many of the terms used to describe exterior wood finishes are used incorrectly, thus adding to the confusion. So let us take a look at the problems facing the homeowner and professional contractor as they try to maintain the natural appearance of wood. In recent articles that compared the performance of various coatings for wood, the performance of clear coatings was often compared with pigmented finishes. To be fair, the pigmented and nonpigmented finishes should not be compared. For example, two years of performance of a penetrating clear finish on a deck would be considered excellent. Two years under the same exposure for a pigmented finish would be considered poor. In comparing clear finishes, it is important to consider the amount and type of pigment that might be included in the finish.

When choosing a finish for wood, there are two elements to consider: aesthetics and protection. These two elements can sometimes be at odds with each other. Aesthetically, there is a growing trend among homeowners to maintain the clear natural look of the wood's original color. However, the best protection from the sun's ultraviolet radiation is obtained from pigmented products, which tend to cover the wood's natural grain and texture. Thus, the problem arises: how to balance the aesthetic desires of the homeowner for a natural look with the protective requirements of the wood surface.

WHY PROTECT WOOD?

Although wood is naturally durable and strong, the effects of outdoor exposure on wood can degrade not only these physical properties but its natural beauty as well. Wood exposed outdoors is subjected to both rain (moisture) and sun (ultraviolet radiation), two major factors that can ultimately cause wood to lose both its natural beauty and strength.

Ultraviolet Radiation

Unprotected wood ages when exposed outdoors. The sun's ultraviolet (UV) radiation degrades the surface of unprotected wood within weeks of outdoor exposure (Figures 2 and 3). The early phase of UV degradation is manifest as a change in color. Dark wood, such as redwood and the cedars, tends to get lighter after initial installation as the colored extractives degrade. Light wood, such as pine and fir, tends to get darker. As the degradation continues, the lignin at the surface [top 75 to 100 [micro]m (0.003-0.004 in.)] begins to degrade, which in turn loosens the wood fibers at the surface. Rain washes the degraded wood fibers from the surface causing the surface to erode. Erosion is more rapid in the less dense earlywood than in the more dense, harder latewood, causing an uneven surface. Mildew growth may also occur as the surface weathers, which darkens the wood's surface. However, in some climates, such as along the seashore, wood tends to weather to a silvery gray. Mildew seems to be less of a problem in these areas. The sun's UV radiation also degrades clear natural finishes.