There are so many cleaners on the market, one for every specific use. We had a pproblem with a hard dinghy that discoloured in a single day while on deck. Apparently there is a microbe that penetrates fibreglass and discolours on exposure to sunlight. We had it and it was awful. Nothing worked. Then I tried Bar Keeper 's Friend. Miracle!

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http://aleriasadventures.blogspot.ie/2014/11/dingy-dinghy-blues-rhubarb-to-rescue.html

Vice Commodore, OCC