You have two main choices of finish: sealers, which prevent wood from absorbing water and delay the weathering process, and stains, which both color and seal the wood. Most people prefer clear finish for redwood or cedar, but if you don't care for the color of your deck, use a semitransparent stain. Avoid direct sunlight when you're working -- and don't apply a new finish if rain is expected within 24 hours.

Test the wood surface by sprinkling water on it. If the wood absorbs water quickly, it's ready to be sealed. If the wood doesn't absorb water, let it dry for several weeks before sealing.

Sand rough or glossy areas and vacuum the deck. Apply clear sealer or stain to all wood surfaces, using a pressure sprayer or paint roller. If possible, also apply sealer to the underside of decking and to joists, beams, and posts.

Use a paintbrush to work sealer into cracks that could trap water, and to smooth out drips and runs. Porous wood may require a second coat of staining sealer for even coverage.