Q My clients have asked me to refinish their older pressure-treated decking, but first I have to clean it. Is a bleach solution safe to use, or are there safer and more effective cleaners? And how about pressure washing—is that a bad idea? I’m worried that it might damage the decking.

A Mark Clement, a PDB contributing editor and a spokesperson for the Softwood Lumber Board, responds: When it comes to cleaning—as opposed to stripping—a wood deck, it’s important to choose the right detergent for the stains and test it on a small area before doing the entire deck. Using a pressure washer is not a bad idea, as long as you use it correctly. Remember, you’re trying to rejuvenate the old finish, not start from scratch.

Bleach. A mild chlorine bleach solution (typically 1 cup of household bleach per gallon of water) applied with a garden sprayer can be used to kill mold and mildew on treated wood. Some industry experts prefer to substitute an oxygenated bleach, like OxiClean (oxiclean.com), for regular bleach because it’s easier on the surrounding landscape. I’ve also been told that borax, baking soda, and even vinegar, which are less toxic to plants, can be effective against mold and mildew, though I haven’t tried them and so can’t personally recommend them. Whatever you choose, use the mildest solution possible, and leave it on long enough to do its job, but not so long that it bleaches the wood. Pre-rinsing and covering nearby plants and the house’s siding will help protect them from the bleach.

Detergents. For general dirt, fading, and other environmental factors, you want a detergent. Some detergents work on mold, algae, and dirt all at the same time—and others may be mixed in with a bleach solution—saving you a step. Read the packaging before you choose one. Detergents as well as bleach solutions need to stay on a deck’s surface for a period of time to actually work, similar to soaking dirty dishes in the sink. Afterward, be sure to rinse the decking thoroughly with clear water.

Pressure washer or scrub brush? A pressure washer can be a fast and productive cleaning tool if used properly, but a pressure-washer wand is not a magic wand. When in doubt, go light, especially with treated SYP decking: 500 to 600 PSI should be plenty of pressure. Use a fan-style tip and keep it between 12 and 18 inches from the decking to avoid damaging it. Keeping the wand in motion, as you would a wood-floor sander, will help prevent the water from etching the surface of the wood.

Depending on the condition of the deck, you may not need a pressure washer. I’ve successfully cleaned outdoor structures with a bleach-and-detergent solution, a bristle brush, and a little bit of elbow grease, an approach that can offer as good or even better results. (For more, see “Maintenance and Restoration of Wood Decks,” Mar/Apr 2007, deckmagazine.com.)

Contributing editor Mark Clement is a deck builder and writer in Ambler, Pa.