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Because the power washer is so versatile and can be used for so many different tasks, they come in many pressure ratings and tip styles depending on their intended application.
I recommend the following pressure and tip selections for cleaning wood:
Use as low a pressure as possible while still being effective;
Pressure for soft wood like cedar or pine should be at about 500 psi to 600 psi, harder woods may go higher but no more that 1200-1500 psi;
Use a fan tip (preferable) or carefully use a rotating tip (shown here);
Always start the water pressure by depressing the trigger away from people and glass windows and at least 24" away from the wood deck, then feathering it into range of about 12-18" from the deck;
As a rule of thumb, try to never get closer than 12" from the wood deck unless you're at low pressure.
Test your pressure and its effectiveness in as inconspicuous an area as possible. Maybe a stair tread. If you mess up and have to replace a piece of wood because you etched it, a stair tread is easier to replace than a deck board.