In the 1920s, Dr. Karl Wolman developed a chemical process for preserving wood: pressure treating. Southern yellow pine, the most common type of wood used for the process, is immersed in a pressurized tank of preservative chemicals. The wood is infused with the chemicals, remarkably preserving the lumber from rot, insects and fungi. Most home builders build decks with pressure-treated wood. Because the wood remains damp for a period of time, the deck should not be sealed until the vapors have dissipated.
Before lumber is pressure-treated, the wood is kiln dried to prepare it for the chemicals and pressurization procedure. In the tank and for a time afterward, the lumber is saturated with the chemicals and water. High grade pressure-treated lumber is dried after the treatment, but most pressure-treated lumber is air dried and bundled together in large pallets. The lumber on the outside of the pallets, exposed to sunlight and air, dries quickly and, in some cases, cracks, warps and twists. The lumber inside the bundle may remain slightly wet, however.
When to Seal
A new pressure-treated deck should be sealed after the wood has dried, but before the lumber dehydrates. A deck that is over-exposed to ultraviolet rays and dry air will crack and splinter. Alan Roos and others from the Forest Products Laboratory recommend that homeowners allow the lumber to air dry for two to three weeks before applying a sealer to the wood. Waiting until the wood is dry allows the sealer to permeate the absorbent pores of the lumber and will improve longevity of the application.
Paint is a poor sealer for pressure-treated wood. Paint is a coating and, while somewhat permeable, paint does not allow the transfer of moisture and air between the wood and the environment. Contractors recommend a pigmented sealer or stain to protect the deck from the drying effects of ultraviolet radiation and abrasive effects of wind, snow and rain. The best sealer for a pressure-treated deck contains pigments or special chemicals that rebuff ultraviolet rays, mildew, molds and water.
Wolman Wood Care Products offers some tips when sealing a pressure-treated deck. Brushing on the sealer is much more effective than spraying or rolling because brushing pushes the sealer more deeply into the pores of the wood. Always test an inconspicuous area for color accuracy. An oil-based sealer gives the deck surface a glossy, professional appearance but may be more slippery than a water-based sealer with its matte finish. Because the underside of the deck is not exposed to sunlight and standing water, only the top of the deck needs sealing. Finally, to keep the deck protected and looking its best, apply the sealer regularly every few years.
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