Last week I shared information with you in this column about using treated wood for various building projects. I shared this information because there has been some recent publicity stating that treated wood may be harmful since arsenic products are often used for these treatments. The Environmental Protection Agency (EPA) has determined that if used properly, treated wood poses no problem to humans, animals, or the environment.

Treated wood is used extensively in our area because of the extreme moisture and humidity problems we encounter, especially during the summer. If untreated wood is used, most outdoor building projects would not last very long.

Many of us have outdoor decks, patios, or other structures that were constructed using treated wood. As stated, we should have no concerns about the treated wood as long as the structures are used properly, but once constructed, the treated wood should be properly cared for.

The performance of wood as a building material for decks depends on its properties, which include engineering properties, finishing characteristics, and maintenance requirements. Poor finish performance, mildew growth, checking, splitting, and decay can be minimized by using the proper finishing procedures. If the deck is properly finished once it is built and maintained periodically, it will look more attractive and last much longer.

Finishes fall into two categories: those that penetrate the wood surface (do not form a film) and those that do not penetrate the wood (do form a film). The primary function of any wood finish is to protect the surface of the wood from natural weathering processes (sunlight and water) and to help maintain its appearance.

The way in which a binder is formulated into a finish determines whether the finish forms a film on the wood surface or penetrates the wood surface leaving no distinct layer or coating. Film-forming materials include paints of all descriptions, solid color stains, varnishes, and lacquers. Penetrating finishes include water-repellent preservatives, semitransparent stains containing pigments, and chemical treatments.

The advantage of a penetrating finish over one that forms a film is that the wood can breathe and the finish cannot peel. Penetrating finishes repel water, thus protecting against the damaging effects of moisture. Penetrating finishes can be used as a pretreatment for other finishes or as the finish itself.

The difference between a water repellent and a water- repellent preservative (WRP) is that the WRP includes a mildewcide which provides mildew resistance. Both finish types contain a water repellent, such as wax, and a binder but do not contain pigments. The amount of the water repellent varies among brands. A WRP is the easiest finish to maintain on a deck.

When pigments are added to WRP solution or to similar transparent wood finishes, the mixture is classified as a semitransparent stain. Addition of pigment provides color and greatly increases the service life of the finish. Semitransparent stains permit the wood grain to show through. Stains penetrate into the wood without forming a continuous film layer and, consequently, will not blister or peel, even if excessive moisture enters the wood. Semitransparent oil-based stains are the next best finish for wood decks.

Film-forming finishes encompass a wide range of finishes. They include latex semitransparent stains, oil and latex solid color stains, paints, varnishes, and lacquers. These film- forming finishes are prone to fail by peeling, cracking, or becoming brittle and are not usually recommended for decks. However, if paint is your only choice because of color limitations or desire for a certain appearance, a few procedures can extend the paint's service life.

The lumber should be finished initially with a WRP and the end grain primed before construction. If highly colored woods such as redwood or cedar are to be painted, they require a stain- blocking primer. Such primers are formulated to prevent the color extractives in the wood from bleeding through the paint.

Though decks are relatively simple forms of construction, they are outdoor structures and are unprotected from the rigors of the weather. Therefore, special attention is required in finishing a deck to assure the best possible performance. Most building supply companies have trained employees who can provide detailed information about what needs to be done to make your deck or patio attractive and last a long time and how you need to do it. You are the one who must see that it gets done.

E-mail address: Everett_Davis@ncsu.edu

Mailing Address:
P. O. Box 2280
Lumberton, NC 28359-2280

Phone: 910-671-3276
Fax: 910-671-6278

Date Created 09/13/01