On white dacron ......
I use a sodium silicate based detergent. This will dissolve/remove all the teeny black spots (actually a fungus) and release most of the air pollution soil, then rinse, rinse, rinse, then apply a solution of oxalic acid to bleach out all the tannin and iron staining, then rinse & rinse again. You dont want any of these strong cleaning agents left on the sail fabric. I do this ON the boat with a spray bottle, a long handled soft brush and a water hose. Do it on a windless, humid, cloudy day. Just dont let the sails dry out, cover the sail/detergent with a plastic tarp to 'soak', let the detergent have enough time to soak, rinse rinse rinse, dont mix the detergent and acid and thoroughly RINSE the sail between the two. Wear goggles, rubber gloves when using strong detergents and oxalic. Oxalic will rapidly adsorb through your skin and can do great harm to your kidneys. Dont use this method on laminated or colored sails, nor do this if you have a teak deck. hope this helps.