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Safely Swabbing the Wooden Deck

House Doctor | Gary Dymski

July 24, 2003

An oxygen bleach cleaner is applied to deck stairs with a pump sprayer to remove stains; this process is gentler on wood surfaces than power washing. (Ken Blazer Photography)

My neighbor is ready to clean his wood deck. He's also about to make a common mistake. "Time for the power wash," he says. "That'll get it good and clean."

Contrary to popular opinion, cleaning with a high-pressure power washer is probably one of the worst maintenance practices for a wood deck.

"A lot of people use power washers or scrub with chlorine bleach," says Tony Bonura, eastern area manager for the Western Red Cedar Lumber Association. "They don't understand that high-pressure washing and chlorine bleach can destroy the fibers in the wood."

There's a better way to keep wood decks clean and make them last longer, says Bonura, who with more than 30 years in the wood business should know a thing or two. He's ready to share some tips - such as how to clean and what kinds of finishes to use - and credits the Forest Products Laboratory in Madison, Wis., for providing him a wealth of information over the years.

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"It's our government dollars at work," he says. The Forest Products Laboratory does not advise against using a power washer or chlorine bleach on a wood deck, and it notes that many contractors use the technique and some type of chlorine bleach. But in both cases, wood can be harmed rather easily.

The Forest Products Laboratory, the public's leading wood research institute, was established in 1910 by the U.S. Department of Agriculture Forest Service. In cooperation with many universities, industries and federal and state agencies, the Forest Products Laboratory has more than 250 scientists and support staff for research in virtually every type of wood use, from pulp to paper to preservation.

It is wood preservation that interests Bonura most, and in decks, one of the first steps is regular cleaning to remove dirt and mildew.

"First, the best way to clean most wood decks is with an oxygen bleach," Bonura says. Several oxygen bleach deck cleaners are available through home centers and hardware stores. Bonura says to check the labels and look for products that contain the highest percentage of sodium percarbonate (when this powder is mixed with water it becomes oxygen bleach). For example, Wolman's makes a specific Deck & Fence Brightener for cedar and redwood, another for pressure-treated wood and another for all types of exterior woods. Each contains a high percentage of sodium percarbonate.

Follow manufacturer's directions, Bonura says. For oxygen bleach products, that means mixing with water and applying with a soft, sponge-like applicator or a pump sprayer. "Scrubbing with nylon brushes can also damage wood fibers," Bonura says. Usually, oxygen bleach should be left on for 10 minutes, then rinsed thoroughly. "A common mistake is that people leave a solution on longer than the manufacturer suggests," Bonura says. "There is a chemical reaction going on with the wood, so following directions is important."

Bonura says oxygen bleach cleaners are best for removing dirt and mold but will not remove stains made by iron and tannin (more common in cedar and redwood decks). For these stains, black in color, use a cleaner with oxalic acid. "Iron stains can look like mold," Bonura says. "So, I tell people if you clean with oxygen bleach, and it doesn't come out, it's probably an iron or tannin stain."

Bonura also advises great caution when using cleaners containing oxalic acid, which also can be purchased at hardware stores and auto parts stores. Oxalic acid-based cleaners are highly toxic and should not be mixed with other ingredients. Again, Bonura says to follow the directions on the label.

Once clean, the goal is to use a preservative that protects the wood from dirt, water and sunlight. There are four common types of deck finishes - clear, tinted, semitransparent and opaque. Bonura recommends a penetrating oil-base stain that is formulated for deck use. The stain should also repel water and protect from harmful ultraviolet rays. "Make sure it is made for decks," Bonura says. "This way it stands up to traffic."

According to research by the Forest Products Laboratory, semitransparent stains penetrate wood without forming a continuous layer. That means there won't be a buildup to chip, peel or crack. In addition, the pigment in the stain also protects the wood surface from sunlight. (Semitransparent deck stains by Cabot and Olympia have received high marks in testing by Consumer Reports magazine.)

How often should you clean and finish your wood deck? That depends on several factors, Bonura says, including traffic, exposure to sunlight and moisture and the type of wood.

"We're seeing that when decks are maintained correctly, you should be able to get more than 2 to 3 years of use before doing it again," he says.

For more information on wood deck maintenance, visit the Forest Products Laboratory Web site (www.fpl.fs.fed. us) or the Western Red Cedar Lumbar Association Web site (www.wrcla. org).

Although he cannot always respond personally, Gary Dymski welcomes letters. Write to him in care of Newsday Home Work, 235 Pinelawn Rd., Melville, NY 11747-4250 or e-mail gary.dymski@newsday. com. Include your community of residence.