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

Water repellents were traditionally formulated with organic solvents; however, contemporary formulations may be water-based or solvent-based, or in some cases, may use paraffin oil with no other solvent. The type and amount of material used to impart water repellence may affect future paintability of the treated surface. Water-repellent sealers designed for use prior to painting have about 1% wax or similar water repellent in a curable resin. Because moisture is repelled, mildew growth may be slowed but is not prevented. These products often provide spectacular beading of water upon initial application. However, they offer wood little protection from the sun's UV radiation or mildew growth; they weather to gray and then black from the growth of mildew, unless a mildew-cide is added to the product (see the Wood-Preserving Finishes section). When water-repellent sealers are subjected to degradation by the sun's UV radiation, the result is a gradual loss of the water beading effect. The life expectancy of these products typically ranges from six months to one year on horizontal surfaces (for example, decks) and one to two years on vertical surfaces. The exposed surfaces of lumber are typically lateral grain. These sealer treatments do not absorb very well into lateral grain; however, they absorb readily into the end-grain of lumber and thus give many years of efficacy preventing end-grain absorption of water. Since the end-grain of wood absorbs water much more readily than the lateral grain, protecting end-grain is more critical to long-term durability than protecting the lateral grain. Treating each piece of wood before it is placed into use ensures end-grain penetration of the sealer.

Wood-Preserving Finishes

Clear wood-preserving finishes differ from water-repellent finishes in that they also contain an appropriate amount of a U.S. Environmental Protection Agency (EPA) approved and registered fungicide. These clear finishes are very similar in appearance to water-repellent sealers. If the fungicide is incorporated to preserve the wood itself, the product must be EPA registered. If the fungicide is present only to protect the clear finish, the product does not have to be registered. The commonly used fungicides are also effective in controlling mildew (mildewcides).

Fungicides are important to all clear natural finish formulations, and most manufacturers incorporate some level of them into their products. The addition of these specific EPA-approved fungicides to the product, in specific quantities, and the subsequent EPA registration, allows manufacturers to make specific claims about controlling decay and mildew when describing their products. Some common fungicides used in these products that can provide decay and mildew resistance to wood include 3-iodo-2-propynyl butyl carbamate, zinc naphthenate, copper naphthenate, bis (tributyltin) oxide (TBTO), N-trichloromethylthio phthalimide, and copper-8-hydroxyquinolate.

Penetrating wood-preserving finishes may also be used as a treatment for bare wood before priming and painting or in areas where old paint has peeled, exposing bare wood. This treatment gives a second "wall" of protection against water that has penetrated the paint film. This is especially effective to prevent absorption of water at joints and at end-grain, thus decreasing the swelling of the wood. As a result, less stress is placed on the paint film and its service life is extended. For treating bare wood, make certain that the manufacturer's label indicates that the clear wood preservative is paintable. Some products have too much wax or other water repellent and the paint will not adhere adequately or dry properly.