As a deck ages, the wood collects tannin stains from leaves and twigs and general grime from the environment at large. As if this weren't hard enough, it also turns gray from sun exposure and develops mold and rot from the rain. A homemade multipurpose deck cleaner needs to include more than one type of cleaner.
Deck Cleaner Ingredients
The first ingredient to include in your deck cleaner is water -- start with a gallon. You'll need the surfactant action of soap, so mix in an ounce of all-purpose, ammonia-free household detergent. Adding 1/2 cup of sodium hypochlorite -- household bleach -- addresses the mold and mildew problems that have developed and prevents their recurrence. Remember never to mix ammonia with bleach. If you want to remove stains and brighten the wood, you need a different solution; add a cup of oxalic acid crystals to a gallon of clear water. Use this solution separately.
Using the Cleaners
Before you mop your cleaning solution on the deck, it should be completely clear of debris; use a vacuum or leaf blower to do this. Use the chlorine bleach solution first, mopping it on the deck and scrubbing it with a hard-bristle push broom. Rinse the deck thoroughly with clear water before using oxalic acid. Allow the oxalic acid solution to soak for 10 minutes or more, scrub with the broom, then rinse it off with clear water.