"John H" wrote
Have you tried oxalic acid yet?

That's the stuff. I get mine in powdered concentrate form from the local
home improvement joint under the name "wood bleach" or something like that.
The reason stains are so hard to get out of fiberglass is that it's porous,
kind of sponge like at a microscopic level and the stains get down into
those little pores. OA will penetrate the pores and remove the stain without
removing any of the material like you would if you tried to abrade the stain
away.