You can hand scrub the deck or you can use a power washer to clean it before sealing the wood. Debris that clogs the spaces between deck boards traps moisture, encouraging mildew and rot. Blast out the debris, using a powerful nozzle on a garden hose, and then push out remaining debris with a putty knife or something similar. Then thoroughly scrub the surface with a sudsy mixture of water and laundry detergent, using a stiff fiber brush on a long handle. Work in small areas and rinse periodically. This may be all it takes to return much of the wood's natural tone.
For a large deck, you can rent a power washer that delivers 1,200 psi of pressure or less to blast and rinse the decking. It should have a nozzle that fans an arc of about 25 to 40 degrees. Wearing safety goggles, hold the nozzle about 6 inches above the deck's surface and spray slowly in line with the wood grain, overlapping your path. Be careful: The powerful spray can erode soft wood grain. After washing, allow the deck to dry for several clear days before applying a finish.
Choose a sealer without pigment if you simply want to seal the deck, or choose a sealer with pigment if you want to change the color of your deck. Be sure to apply according to label directions.
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