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, then push out remaining debris with a putty knife or an old handsaw. 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 25º to 40º. 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.