Davis has a product called FSR. It stands for Fiberglass Stain Remover, and it is a blue goop, that you apply, wait and hose off. It is made for rust stains and the like. It is the only thing that I have seen remove the light brown tannin stain that you get sometimes at places like Barnegat Bay.
You can buy it as West Marine. I would bet that it will work. Be careful to hose it off real well. If it runs down, it will remove stains that you didn’t know you had and make lighter “drip” marks where it runs downhill.
Phil Scheetz
FS 4086