Another solution to nasty sail stains...
With various stains on sails I found the best solution to remove stains on sailcloth is a mixture of laundry detergent or dish soap and Oxy-type cleaner. Be sure to mix with hot-warm water until the Oxy crystals have disolved. Using a dabbing motion with a wash rag, start small and work your way around the stain. Let set for 3-5 minutes and rinse. Be sure to rinse the area with clean tap water...if you use river/lake/ocean water...you could restain the cleaned area. This solution also works on deck stains and does wonders on dark bird droppings. You can use cheap Oxy cleaners...Big Lots as some for as little as $1.99....or if you recently won the lottery...buy new sails!;D