Five months after carpenters screwed down the last board on my oak and mahogany deck, I rented an industrial pressure washer, cranked it wide open and learned too late why wood manufacturers warn people to think before they shoot.

I made two key mistakes. I applied a harsh wood cleaner for longer than directed. I also held the pressure wand -- rated at 3,200 pounds per square inch -- too close to the deck.

The next day, the deck's almost-new surface resembled an ancient river bed crisscrossed with furry strips of raised wood grain. A subsequent coat of cedar-toned stain made a dozen ugly gouges glare like beacons.

I cursed myself, but experts say pressure-washer mishaps are common. People ignore user manuals and damage costly decks, siding, brick or driveways. Sometimes it's worse. In 2004, an estimated 3,747 Americans needed hospital care for pressure-washer injuries, with 20 percent sustaining chemical burns to their eyes, according to the Consumer Product Safety Commission.

Manufacturers say consumers need to know the operational basics, as that can spell the difference between a job well done and a frustrating or dangerous experience.

Electric or Gas?Both electric and gasoline-powered pressure washers excel at removing grime, oxidation and stains. Gas units are more powerful, typically generating 2,000 to 4,000 pounds of pressure per square inch (PSI) compared with 1,300 to 1,800 PSI for electric units. Both types come with adjustable nozzles or graduated tips that alter the spray pattern from pencil point to wide angle. The larger units can produce a water jet strong enough to cut through a cement block. Electric pressure washers are fine for "light-duty" homeowner applications, said Powermate Corp. spokeswoman Nicole Toledo. Electric units are also easier to tote around and cost less than $170, or about half the price of gas units. But gas units "are the obvious choice," she said, for large projects.

In July 2005, Consumer Reports agreed, finding that gas units can clean a dirty patio three times quicker than the fastest electric units, mainly due to a larger, stronger spray pattern. Homeowners with frequent pressure-washing chores may find purchasing makes more sense than renting, the publication said. United Rentals in Gaithersburg quoted $65 for a one-day rental of a 2,700 PSI gas unit.

Easy Does ItFaced with a big project, it's tempting to rush things by pressing the water nozzle too close to surfaces. Don't. Pressure washers -- gas units especially -- can punish operator error by splintering and etching wood.

Marc Cantor, owner of Potomac Pressure Washing, has seen a multitude of do-it-yourself bloopers, including "blown-out [rail] pickets," raised wood fibers that give the deck surface a permanent furry look and "scars all over" the deck, he said. "You can definitely see the difference between somebody who doesn't know what they're doing and somebody who does."

Powermate engineer Kurt Beckman suggested testing the pressure washer on an inconspicuous spot, at low pressure. If the dirt isn't removed, move the nozzle a bit closer, but keep it within the manufacturer's specifications. Using excessive power "will rip right through your siding or wood," he said. "You could literally strip the paint right off your car." Beckman recalled that his first use of a pressure washer resulted in "holes a quarter-inch deep into my [deck's] wood before I realized it."

Cleaning DecksPowermate recommends a four-step cleaning process for decks: Remove all items from the deck and rails, apply an approved detergent, power clean the surfaces, and rinse. Start cleaning "at the back of the deck and walk your way across, blowing the dirty water over the dirty [portion of the] deck," Beckman said. Use a wide-angle tip. Moving the nozzle in a side-to-side sweeping motion should lessen uneven cleaning or scarring. Using an approved cleaner will help lift dirt and eradicate mildew spores. After rinsing, wait for the deck to completely dry -- about 48 hours in warm, dry weather -- before staining or sealing it.

Cantor had additional advice. "Use a high dilution of water with whatever chemical you're using and constantly keep [the wood] wet. If you let it dry, it's going to destroy the wood [by drying] it out so much faster than you were prepared for." Cantor has seen novices clean decks with straight bleach, resulting in surfaces "dried out like a piece of fruit."

Cantor said a professional can clean the average 15-by-20-foot deck with stairs in about three hours. The undersides are not typically cleaned. He said his firm would charge about $350 to $375.

MaterialsAvoid pointing the unit's tip underneath vinyl or aluminum siding. Doing so might blow the siding off or trap moisture underneath, creating a breeding ground for mildew and mold, Toledo said.

Cantor agreed. "We're totally against cleaning gutters with pressurized water because water can go underneath your shingles," he said.

Aluminum siding requires special precautions "because it's painted and you can strip off the paint," Beckman said. Harsh cleaning agents can discolor painted aluminum surfaces.

Toledo said many consumers are unsure about the proper cleaning of composite and exotic decking materials. Comparing them to "an exotic car," she said, "Our units are safe to [clean them] but, again, you have to be very careful." Use a low-pressure nozzle, stay at a safe distance and test an inconspicuous area first. When in doubt, contact the manufacturer or distributor, or hire a professional.

Prior to a production ban in January 2004, manufacturers of deck and playground equipment injected chromated copper arsenate -- an arsenic-based pesticide -- into pressure-treated lumber. Controversy continues over the proper care of this aging wood.

The District-based Environmental Working Group advises people not to pressure-wash CCA-treated wood, as the spray can spread residual arsenic. Instead, it recommends either replacing high-traffic surfaces with newer lumber treated with alkaline copper quaternary, or ACQ, or cleaning the deck with a soft brush. Seal it every six months. If a power washer is used, the Environmental Working Group suggests directing the spray away from gardens and play areas.

The Consumer Product Safety Commission suggests that people concerned about their CCA-treated decks manually clean them with soapy water and brush and seal them annually.

SafetyThe top of a slippery two-story ladder is no place for an introductory lesson in pressure washing, said Mary Pat McKay of the American College of Emergency Physicians. The water's backward thrust could push a person off a ladder.

Accidentally raking the jet stream across a bare leg can cause a laceration or puncture wound. Other injuries are caused by forcible injection of cleaning fluids "deep into soft tissue," McKay said. "The tissue can just die because it's sort of blown apart by the pressure and then by the presence of the [cleaning] fluid in the tissue."

McKay suggested taking sensible precautions. Wear eye protection, keep water away from electrical sources, and don't operate a gas unit inside a home or garage. Damaged high-pressure hoses should be replaced, not repaired. Do not attempt to clean pets using a pressure washer.

Powermate's Beckman said only approved cleaners should be used in pressure washers. He recalled one incident in which a man tried to pressure-wash his car's engine using gasoline.

"Guess what?" Beckman said. "Now you've made a flamethrower. That's one of the warnings in our manual."