I would start with wiping down with acedtone
or interlux fiberglass cleaner. My next step would be to wet sand with 600 or finer. This will tell you how deep the discoloring is. If the brown color remains your only option is to have it redone.Could this area have come in contact with fuel said from and over flow.
diesel inspection/cleanout port
It finally happened. My gunky fuel got to the engine. Anyone know what kind of ports I can install in my fiberglass fuel tanks that will not leak and hold up to the sloshing diesel? What about bedding and sealing?