To remove a mild case of mildew, scrub with a mild cleanser or detergent. Next rinse with a household bleach to kill surviving spores. Lastly, rinse with water. When applying a new finish, be sure it contains a mildewcide. Household bleach should never be mixed with detergent containing ammonia.
Fumes can be fatal.
For severe mildew infestations, scrub with a stiff bristle brush using a solution of one cup of trisodium phosphate, one cup of liquid household bleach and one gallon of warm water. If necessary, follow with an application of 4 ounces of oxalic acid crystals dissolved in one gallon of warm water in a non-metallic container. Apply evenly with a soft brush. When wood dries, rinse with water. Oxalic acid is poisonous, but not dangerous it precautions are taken.