This is an ideal time to clean and seal your wooden deck. Removing accumulated grime and mildew is best done on cooler, cloudy days that allow the deck to stay wetter longer and give the cleaner time to work.
* Need to know: The square footage of the deck, including stairs and railings, to figure out how much cleaner and sealer you need. A gallon of stain or sealer typically covers 150 to 300 square feet of deck. (Commercial deck cleaner typically lists square-foot coverage on the package.)
* Good advice: If screw or nail heads have popped up above the surface of the deck, make sure they're back in place before you start your work, to avoid injury and tearing cleaning implements.
Apply cleaner, available at home centers and online, with a paintbrush or a sprayer. Let the cleaner work into the deck surface for about 15 minutes, then remove the residue with a squeeze mop; dirtier decks may require more than one application. Don't scrub, even if dirt and mildew are firmly imbedded, to avoid gouging the softwood used in most decking.
* Weather wise: Allow two or three days of good drying after cleaning before applying a waterproof sealer. Mild weather (55 to 70 degrees) is perfect. If it's too hot and dry, the sealer will dry too quickly and won't permeate the deck surface. If it's too warm and humid, the sealer will take too long to dry, for a greater chance that wind, leaves, animals and rain will disturb the surface. That can result in an uneven coating.
* Be sure to ask: Some experts advise against using power washers on deck surfaces, because pressure can damage softwood. But using a power washer saves time. Before you rent one at a home center or rental outlet, ask what you should use. The appropriate washer for this kind of job typically delivers 1,500 to 2,100 pounds of pressure per square inch.
* Bad advice: "This cleaner won't harm plants." Even if the manufacturer's label tells you that, cover plants under or near the deck with plastic sheeting, or soak the plants with enough water to neutralize any cleaning solution that spills onto them.
* What it will cost: Bleach and water is the cheapest deck cleaner. A quart of liquid bleach will be more than enough; use a mixture of three parts water to one part bleach. Deck cleaners available at the home center come in powder and liquid and cost $5 to $15 for 16 to 32 ounces. Stains cost $15 to $25 a gallon. Water-repellant sealers run $15 to $20 a gallon.
* Jargon alert: Deck sealers can be water repellents or water-repellent preservatives (WRPs). Water repellents are either water-based or oil-based; the latter use organic solvents such as mineral spirits or turpentine. Whether water- or oil-based, the products typically are mixed with linseed oil or varnish or paraffin wax, which is why wood on a treated deck can feel like a candle.
WRPs add a mildewcide, an ingredient designed to limit the appearance of mildew, for better performance. They take a short time to apply -- maybe only an hour -- and last one to two years.