Photo by Crandall and Crandall
COLOR PIGMENTS in semitransparent and solid-color finishes help protect wood from UV rays. Use finishes designed for decks; products meant for siding won't stand up to foot traffic.
Deck finishes fall into two categories: sealers and stains. Both are formulated to seal out the elements. As their name implies, clear sealers are nonpigmented finishes. Stains are available with a little pigmentation (referred to on the label as "tone"), semitransparent, and in solid colors. Unlike paints, which form a surface film, clear and pigmented finishes penetrate the wood and can stand up to foot traffic.
"The ideal deck finish does three things," says Charles Jourdain, vice president of technical services at the California Redwood Association, a lumber-industry trade group. "It repels water, preserves the wood with a mildewcide, and screens out UV rays." Some products waterproof only. For maximum protection, the label should list all three features.
Oil- or water-based? Deck finishes are either oil- or water-based. Which you choose depends on your priorities. According to Mark Knaebe, a chemist with the U.S. Department of Agriculture Forest Products Laboratory who evaluates deck finishes, oil-based finishes provide more and longer-lasting protection. "Oil-based finishes penetrate deeper into the wood than water-based finishes," Knaebe says. Familiar names here include Cabot's, DAP, Flood, Olympic, Thompson's, and Wolman.
On the other hand, water-based products are easier to clean up than oil-based products. They're also more forgiving in damp conditions, explains deck-maintenance contractor John Sundquist, of Northbrook, Illinois, based American Deck Maintenance: "While the wood surface must be bone-dry before it accepts an oil-based sealer, damp wood can absorb a water-based product." Water-based finishes also last longer than they did just a few years ago. Companies like Wolman and American Building Restoration Products (X-100 Natural Seal) make a water-based version of their product.
Clear or color? Clear deck finishes are popular because they allow the natural grain of the wood to show through. And because they're transparent, you can't leave any lap marks during application, which is a common problem with pigmented finishes. But, they aren't as good at blocking UV rays.
The best UV protection comes from a combination of chemical inhibitors and color pigments. Though inhibitors in some clear finishes slow UV penetration, they tend to break down relatively quickly. The result is that most must be reapplied yearly.
Lightly pigmented and semitransparent finishes add color to the deck while allowing some of the grain to show through; they also form an effective UV barrier. The more pigment the finish has, the better it is at blocking UV rays. That's why a semitransparent deck finish will last up to three years or more before another application is needed.
Solid-color finishes offer the most UV protection. Unfortunately, most aren't formulated for foot traffic. If you want a solid color, look for products made expressly for deck surfaces, such as Cabot's solid-color decking stain with Teflon.
For do-it-yourselfers, there are two drawbacks to pigmented finishes. They leave lap marks or areas where the finish is uneven if you don't apply them carefully. They also show traffic patterns as they wear, which requires feathering extra finish into affected areas during reapplication.
Figure on spending $15 to $25 per gallon for a good clear or pigmented deck finish. Finishes that cost less probably won't provide all of the protection your deck needs.