Pressure washers can be great for cleaning large areas, but they're not foolproof. Before grabbing a high-powered pressure washer and attacking a dirty wooden deck, you need to practice a little on surfaces that aren't as delicate as deck lumber.

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All pressure washers work on the same principle: You connect them to a water source such as a garden hose and the pressure washer drives the water through a nozzle at high pressure. Typically, a pressure washer will have a system that allows the user to pump detergent through the nozzle, and the nozzle opening can be adjusted from a needlelike stream to a broad spray.

Pressure washers are rated by two measures: pounds of pressure per square inch they generate, and gallons of water per minute they pump. Multiplying the two measures gives you a power rating that helps compare one machine to another. A pressure washer that generates 2,000 psi and pumps 2.0 gpm has a power rating of 4,000, for example.

Choose a pressure washer by the jobs you have in mind. Lightweight pressure washers offering less than 2,000 psi are good for washing the car or cleaning lawn furniture, but they don't have the power to make quick work of a big job like washing all the siding on a house.

On the other hand, washers generating more than 3,500 psi are probably more than a typical homeowner will need. Not only is such a machine expensive to buy, it also may be so powerful that it will be unsafe to use.

Tips for use

Shain Brady is a product manager for TTI Outdoor Products, which markets the Ryobi pressure washer that went on sale this year at Home Depot stores. Brady said it's critical for first-time pressure washer users to read the manual carefully before using the machine.

"If you're washing a car, for example, and you get too close, it's easy to damage the finish or knock off a piece of molding," Brady said. "People used to using a garden hose don't have a concept of the pressure of the water coming out of a pressure washer."

Pressure washers can be adjusted to go from a broad, fan-shaped spray to a powerful, pencil-thin stream. The narrower the stream, the greater the scouring effect you get and the greater the potential for damage. In most cases, the angle of the spray is adjusted by changing a nozzle at the tip of the spraying wand.

Start jobs with the nozzle held at least 2 feet from the surface. Move in gradually until you achieve the cleaning effect you want, but generally, you should be no closer than 12 inches away.

Pressure washers come with attachments that allow you to add detergent to the spray. Use only detergents specifically designed for pressure washers.

Pressure washers often are used for deck cleaning, but that's a job you must approach carefully. It pays to clean a driveway or sidewalk first to get used to cleaning without applying damaging pressure. And if you're using a cleaning agent such as an oxygen bleach, read and follow the directions carefully.

Generally, it's a good idea to wet the deck with a broad spray under no pressure or light pressure, then apply a cleaning agent. Allow the cleaning agent to work a few minutes, then use the pressure washer, set on narrow spray, but not pinpoint spray, to loosen grime.

Start in an inconspicuous area to be sure you can control the spray enough to clean without blasting away chips of wood. Use broad, sweeping strokes following the grain of the wood. Overlap your strokes over areas just cleaned as you advance across the deck.

Rinse with plain water and allow the deck to dry a couple of days before resealing, if resealing is needed.

Cleaning painted house siding prior to repainting is also a pressure washer job that requires proper technique. If you angle the nozzle upward, it can force water into the gaps between boards, which is bad for the siding.

Try to spray boards from the same level or a little above.

You also must be careful when spraying around windows and light fixtures. A high-pressure spray can break glass or blast parts loose.

James Cummings writes for the Dayton [Ohio] Daily News.