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

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In the absence of a mildewcide, wood that gets wet or damp (even for a short period of time) may develop mildew. As the mildew fungi grow, they become visible because they often develop highly colored pigmentation. Gray or black coloration is the most typical, but various other colors commonly occur. The mildewcides found in paints and stains are not very effective in destroying pre-existing mildew. Therefore, it is important that mildew be completely destroyed prior to staining or painting.

CLEAR NATURAL FINISHES

Stain and paint manufacturers have produced a number of clear products to meet the aesthetic desires of the homeowner while meeting the requirements of wood protection. Clear natural finishes generally fit into four categories: waterproofing, water-repellent sealers, wood preservatives, and UV-resistant clears (blocking or absorbing). Some clear natural finishes on the market today may be combinations of these categories. High solids, water-based, oil emulsion, and traditional solvent-based formulations are just a few examples of the different types of clear finishes sold within each of the categories of clear natural finishes. Air quality regulations have limited the amount of volatile organic compounds (VOCs) permissible in paints, which has motivated paint manufacturers to formulate an abundance of new water-based clear finishes. Another reason for the abundance of new finishes is consumer demand. Some of these new clear natural finishes may be film-forming, and therefore, they may not penetrate the wood's surface very well. Clear finishes also degrade from UV radiation and moisture. For penetrating finishes, this degradation is not serious because the wood can be refinished without extensive surface preparation; film-forming finishes often require extensive surface preparation, even to the extent of complete removal of the old finish, prior to refinishing. Thus, when choosing a clear product, it is important to match the aesthetic desires and the exposure to the weather with the protective requirements of the wood and, at the same time, to understand the performance limitations of the finish.

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Waterproofing Products

Waterproofing clear products are often confused with water-repellent products. Waterproofing products form a membrane designed to withstand hydrostatic pressure (like wind-driven rain) and keep water out of a home throughout long periods of wetting. Waterproofing coatings are often elastomeric and are designed for brick, stucco, and other masonry surfaces, not wood. They are products used around flashing to completely stop water absorption.

Water-Repellent Sealers

Water repellents shed water for short periods, like rain or water from a sprinkler. They are not completely impervious to water absorption. Clear products that fall into the water-repellent category are the penetrating finishes. They typically contain a small amount of wax (1-3%), usually paraffin, or other water-repellent material that decreases the amount of water absorbed into the wood, thus decreasing dimensional changes, warping, and splitting. They also contain an oil or resin that helps seal the surface and a solvent to facilitate absorption of the other ingredients.