Practice

PRACTICE
If you don't power wash correctly, you can damage wood. Practice on old boards before doing your deck or fence.
TECHNIQUE
The wand should always stay in motion when it is close to the wood. Stopping the wand while spraying will scar wood. Arc the wand up and away from the surface at the end of each stroke. Use wide strokes. Stand closer for chromated copper arsenate and other treated lumber. For softer wood, such as cedar and redwood, stand farther back. Avoid spraying windows and painted siding.
PRESSURE
The most pressure you want to use on soft woods such as cedar and redwood is 1,500 pounds per square inch. Deck restoration contractors often recommend using cold-water power washers, with pressure set at 2,800 to 3,200 pounds per square inch, for CCA-treated wood. Hot-water power washers, using 2,000 to 2,800 pounds per square inch, can be used on grease and chemicals.
NOZZLE TIPS
Four tip sizes are usually used for power washers. The numbers refer to the degree of spray dispersal:
0: The spray pattern is a concentrated straight stream. Never use this tip for your exterior wood because it will cause damage.
15: Use for tough cleaning jobs. It's good for deck cleaning.
25: Use for modest cleaning jobs. It's good for fences.
40: Use for light cleaning jobs. It's good for more frequent cleaning jobs.
HORSEPOWER
A commercial-grade power washer with pressure of 2,800 to 3,200 pounds per square inch should have a 9- to 11-horsepower motor.
HOSES
Power washers typically come with 50 to 100 feet of high-pressure hose. If you need to get the machine closer to the surface to be cleaned, it's usually best to add more supply hose (use a five-eighths-inch garden hose).
WATER SUPPLY
Most power washers require 3 to 5 gallons of water per minute to operate. Know what the requirement is for your unit. City water systems usually have enough water pressure to supply power washers.
Sources: Do-it-yourself author James Dulley,(www.dulley.com); Dave Sullivan, Wood Re New