if the sails are *WHITE* woven *DACRON*
Get some SodiumSilicate based detergent (usually found at janitorial supply or industrial hardware stores .... sometimes boat chandlieries will carry a product branded "tuff-eNuff"), then spray on and mist with a little water, scrub lightly with a long handled brush with SOFT bristles, then cover with a plastic tarp to 'soak'. If you cant find sodium silicate detergent, then TSP will serve --- take a small bucket with HOT water and dissolve the TSP into the hot water until you can dissolve anymore, then bursh-on as above. After soaking for about an hour, respray and rebrush and let soak under a platic tarp (to prevent the detergent from drying). Then scrub the sail and rinse thoroughly with a garden hose to be SURE that ALL the detergent is rinsed out. You may have to repeat with 'stubborn' staining. The soak time is the most important. These detergents are caustic and the caustic will **dissolve** the mold/mildew cells.Then get some oxalic acid crystals (hardware or paint store --- also known as 'wood bleach'). Dissolve the oxalic in a small bucket of HOT water .... just enough water to dissolve ALL the crystals. Apply the oxalic with the soft bristled brush to remove the 'shadows' of the mold mildew, the tannin stains from dirty water, etc. Wear gloves, foul weather gear, etc. as oxalic will absorb through your skin and harm your kidneys. Then *thoroughly* rinse the oxalic from the sails.Best to do this ON the boat on a cool cloudyu windless day --- , slowly raising the sails a few feet at a time to apply the chemicals, brush the crud and rinse with a garden hose. Dont use these chemicals anywhere near BARE TEAK as they will extract the tannins from the teak and that will get all over the sails. Do NOT use 'organic solvents': gasoline, kerosene, toluene, paint thinner, etc. as they may dissolve/soften the polyester and the 'filler' thats held between the fibers. after thorough rinsing, raise the sails all the way to dry .... then stand back and admire your almost totally white and cleaned sails. hope this helps.