Actually, if you want to kill the mold
spores, and bleach hasn't done it, consider renting
a hand steam cleaner. It might work where all else has failed. Once it's sterile, you can do whatever you want to the area.
Incidentally, you said it is in fiberglass
, are you sure? If it is gelcoat
, that is different. Gelcoat
is microscopically small bubbles, and if something (like rubbing compound) takes the tops off the bubbles, then openings then attract all kinds of filth. Usually, for that, an acid wash, followed by a polymerizating compound will first clean, then seal it.