Remove any paint from the infected area with a scraper, sandblaster or steel-bristled brush. Wear an OSHA-certified breathing mask to prevent a possible lung infection from the mildew or algae.
Dry the wood siding as thoroughly as possible. Use a large industrial fan, if necessary, to circulate air across the wood for several days if the area is enclosed.
Put on protective gloves. Mix 10 tablespoons of trisodium phosphate in 1 gallon of water.
Soak the bristles of a stiff brush in the mixture and scrub the infected areas until it is clean of all mildew or algae. Let the area dry and then brush it again with the chemical to ensure that all the growth is stopped.