What I do for dirty and mould stains is get a garbage wheelie bin, put the sail in and chuck in two tubs of NappiSan or similar and fill with water. Leave for a week in the sun, shake often. Works petty well
“...
Mildew is not a structural problem.
Mildew is ugly.
Sail owners get a little uptight when their sails turn black and splotchy ...
... In our experience, once a sail has had a mildew problem, it will probably always have a mildew problem. A good cleaner can reduce the black spots to dirty white spots, but if the boat stays in the same area where the mildew started, the spots will soon be gray, and then it’s only a matter of time before they are black again...”
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Gord May "If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
i miss the days of small boats. with my first boat (23ft) i used to take the sails off each season and take them in the shower with me to clean them ha ha
The answer is two-step. Physical dirt removal with any of many cleaners, and then percarbonate (Oxiclean) for the stain. Read the above article. Safe for most sail fabrics if used as directed. This is what the pro sail cleaners use. Also, don't aim for perfection on the first go. The Oxiclean will weaken the mildew stain and the sun will often finish it off.