I can't remember, or find the recipe, but I had been given directions once for a solution of Bleach, Tide detergent, and Spicand Span, diluted in a gallon of water. I would spray it on with a pump sprayer, let it set about 10 minutes, then rinse it off with a hose. The black mildew would just run off in sheets. Good stuff. You should be able to find a suitable "recipe" on the net.