MEET YOUR ENEMIES
Photo by: courtesy of PPG
Besides enhancing a deck’s aesthetic appeal, a protective finish must defend against water and sunlight. Make no mistake that these two elements are among the most destructive forces in nature’s arsenal, and in many climates, they are at work on a day-to-day basis.
When someone talks about wood being damaged by sunlight, they’re really talking about damage from ultraviolet (UV) rays. Wood is essentially a combination of cellulose fibers and lignin, the glue that holds together those fibers. UV light is especially good at destroying lignin, and as that happens, the cellulose fibers loosen and wear away. This exposes a fresh layer of fibers and lignin, and the process repeats.
To be effective, a deck finish either must reflect or absorb this harmful UV light. This applied protection, though, sits on the top layer of the wood, where it’s susceptible to wear and tear. Reapplication—as often as once a year—is usually necessary. If you go without reapplication for too long, you will be staining only the cellulose fibers that are soon to slough off. That’s why lightly sanding an old deck before refinishing is best practice.
Wooden deck planks come from trees, and trees love water. They suck it up at any opportunity. As wood takes on water, it expands across its width (perpendicular to the direction of the grain). As it dries, it shrinks. The cycle will be slower in dense decking (such as ipé) than in soft decking (such as cedar).
Decks in many parts of the country are exposed to frequent wetting and drying cycles. Board ends are most susceptible to this expansion and shrinkage, which is why you often see small cracks at these areas. More cracks mean more water absorption, and the cycle continues.
Penetrating deck finishes aren’t designed to waterproof a board completely, but rather to slow the wetting and drying so that the wood is less likely to develop cracks from dramatic swings in moisture content. Coating the ends of each board with finish is an essential part of sealing a deck.
The moisture level is also a factor in the growth of fungus on deck planks. Finish manufacturers typically keep this maintenance headache under control with the help of mildewcide additives.