Purchase a pre-mixed oxalic acid deck cleaner or purchase oxalic acid crystals from a hardware store or home-improvement center and mix a solution of four ounces crystals to one quart water in a non-metallic container.

1) Wearing protective glasses, rubber gloves, and old clothes, apply the acid solution to a one board at a time and scrub with a soft nylon brush attached to a broomstick-length handle.

2) Let a section dry, then rinse with clear water.

3) Allow the deck to dry for several days before applying any type of protective finish.