How To Clean Cracks In Deck Boards

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June 3, 1993

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June 3, 1993

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DESPITE their generally sturdy construction, most wooden outdoor decks begin to look shabby after only a few years without care, as mold, mildew and rust set in.

Simply washing by hand can brighten a deck that is not too dirty. Use a stiff-bristled scrub brush dipped in a solution of warm water and strong household detergent. Follow the label's directions for heavy-duty cleaning, or use about half a cup of detergent for each gallon of water.

Wash the deck on a cloudy day or work on one relatively small area at a time so the solution does not dry on the wood. After scrubbing, rinse the deck by mopping it with clear water or by spraying it with a garden hose.

It is important to keep the spaces between the boards free of debris. The spaces allow water to drain quickly from the deck and provide thorough air circulation, which retards decay and mold growth.

To clean between deck boards, use a putty knife or an old handsaw. If the spaces between boards are too narrow for either tool, consider widening them with a circular power saw. Set the saw's blade depth to the thickness of the deck boards, and make careful passes with the saw along the seams. Use a heavy-duty blade with carbide teeth to produce a gap of about three-sixteenths of an inch.

If you have a large deck, renting a power washer to clean it once or twice a year may be a better idea than scrubbing it by hand. Power washers save a great deal of effort and also clean between the deck boards. Rental stores carry this equipment and should provide instructions for its use.

Rent a washer with a pressure rating of about 2,000 pounds a square inch. Choose a nozzle with a relatively wide arc for general cleaning (40 degrees is a standard size), and a narrower one (usually 15 degrees) for cleaning between the boards. Be careful where you aim the narrow nozzle -- the stream can break a window or dent aluminum siding -- and avoid nozzles narrower than 15 degrees; both sizes can damage wood if aimed at one spot for more than a few seconds.

Wear safety goggles when operating a pressure washer. Spray in line with the wood's grain, holding the nozzle at a slight angle about six inches above the deck's surface. For even cleaning, work slowly and at a uniform speed; successive parallel strokes should overlap.

Very dirty decks may require washing with deck cleaner, which is available at hardware stores. Most cleaners are caustic compounds that work by eating away the surface of wood to expose fresh wood underneath. Be careful to follow the directions and safety precautions listed on the container.

Goggles, rubber gloves, sturdy shoes, and clothing that covers your arms and legs are necessary protection from splashes, which can cause severe irritation. Protect nearby plants and grass by covering them with plastic sheeting.

After mixing the cleaner with water, as directed, apply it with a mop, paint roller or garden sprayer. Allow the cleaner to work for about 15 minutes; then, scrub the deck with a scrub brush and rinse it with hard spray from a hose. Two or more applications may be needed to remove deep stains and to achieve a uniform color over the entire surface.

Deck cleaners may darken some wood. To lighten it again, or to bleach out rust or other stubborn stains, wet the wood with water, and then mop it with a 5-percent solution of oxalic acid, which is available in that strength at hardware and paint stores. Follow the same precautions as in applying deck cleaner.

Leave the solution on the wood, for several hours if necessary, until lightening occurs; renew the solution if it dries. Then, rinse the wood thoroughly with plain water.

Most new decks and those treated with deck cleaner need some type of finish for protection, if not for appearance.

At the very least, regardless of whether the wood is new or freshly cleaned, apply a clear water-repellent like Thompson's Water Seal, Cuprinol Clear Wood Seal or Wolman Raincoat as soon as possible after the wood is dry. Water repellent protects wood against cracking, splintering and warping caused by moisture.

Even naturally decay-resistant woods, like redwood and cedar, and lumber that has been pressure treated with chemical preservatives, are subject to moisture damage.

With new decks made from pressure-treated lumber, waiting at least a month before applying water repellent is often advised to help the wood absorb the repellent. The best strategy, though, is to apply it immediately, but only as much as will soak in. Then, wait a month before applying additional repellent or another finish. (If a new deck becomes dirty by the end of the waiting period and you desire a fresh-looking surface, clean it with deck cleaner.)

Simple water repellent will permit new or cleaned wood gradually to turn gray, a process caused chiefly by sunlight. But to slow the change (no treatment can prevent it) use a repellent that contains an ultraviolet inhibitor. To speed the change -- for instance, to give new decking an older appearance -- mop the deck with a mixture of one cup baking soda to one gallon of water after washing but before applying repellent.

To prevent decay from mold, mildew and other biological sources, apply repellent containing preservative. (This step is not needed on pressure-treated lumber.)

Deck-finishing products are available containing all these protective ingredients, either as clear finish or with a stain. Stain should not be applied more often than once every three years, or the surface may become muddy.

For the best protection against damage by termites and other insects, choose a naturally resistant species, like redwood or cedar, or be sure that your wood has been pressure treated. Only one readily available preservative also contains insecticide that will kill these pests: Cuprinol No. 10 Green Wood Preservative. Its active ingredient is copper naphthenate, which gives wood a greenish tint.