Clean them ON the boat .......
Here is a repeat of a posting of about 2 months ago Clean them ON the boat .......For woven white DACRON sailcloth ONLY: On a cloudy windless day using spray bottles of a sodium silicate detergent and a garden type water hose, slowly raise the sail a few feet at a time as you spray on the sodium silicate applying until the sail is fully raised. Use a long handled soft scrub brush to help spread the detergent. Drop the sail and cover with a plastic tarp to prevent the detergent from drying (mist with water if necessary). After about 20 minutes to allow the detergent to 'work', slowly raise and scrub and mist the sail then drop to the deck and recover/soak. Raise, etc. again then *thoroughly rinse* several times until ALL the detergent is removed. This should remove most of the "black crap": artillery fungus, air pollution specks, etc. The sodium silicate will dissolve most of such 'dirt'. You can get strong sodium silicates at a janatorial supply. Wear goggles, etc. when using. For tannin and rust stains, mix up a solution of oxalic acid (hardware/paint store stuff) and spread over the areas to get rid of the 'brown'. Rinse thoroughly. Caution: Oxalic will rapidly cross your skin and accumulate in your kidneys doing great potential harm .... wear gloves, goggles, etc. DO NOT mix sodium silicate and oxalic. Then *thoroughly* rinse the sail. Let the sail thoroughly dry (go sailing). The sodium silicate will strip all the old dead wax in the gelcoat where it splashes down from the sail, so you'll need to rewax the boat. The old dead wax accelerates gelcoat oxidation so it should be stripped every 2-3 years anyway. Use the oxalic on the gelcoat to remove tannin and rust stains ("moustache", etc. at the waterline, etc.)Dont use this method on laminated or colored sails. Hope this helps.