Sand your deck with an orbital sander to remove the old finish. Start with an 80- to 100-grit sanding pad and sand until the wood is bare. Sweep away the dust, then sand the deck again with a 180- to 220-grit sanding pad. Use a handheld sander for vertical and narrow surfaces, such as the railings and steps.
Sweep away the dust from the sanding process. Scrub the deck clean with a solution of 1 cup of trisodium phosphate mixed with 4 gallons of hot water. Rinse the deck thoroughly and allow it to dry for 24 hours.
Apply exterior wood stain with a roller or paint sprayer. Use a brush or rag for staining narrow surfaces. As you work, have a partner wipe up the excess stain with a clean cloth. When staining and wiping, always work with the grain of the wood. Allow the stain to dry.
Apply an exterior polyurethane sealer to the stained wood. Use a lamb's wool or foam applicator to apply the polyurethane — this type of tool will not leave brush marks in the finish. Allow the polyurethane to dry.
Buff the surface of the polyurethane with a 400-grit sanding pad. You only need to scuff the surface slightly to help the subsequent coats of polyurethane bond to the hardened coats — do not sand the polyurethane away. Wipe the deck down with a damp cloth after sanding. Apply up to five coats of polyurethane.