Last month I had a similar problem with the fiberglass tank on my '68 fastback, I had sealed it years ago, before the ethanol gas, and now it was dissolving the sealer. I recently stripped out the old sealer with solvent, and re-sealed it. It was a fair amount of work to remove the old sealer. I had talked with Bill Hirsch Auto, who had made the old sealer, as well as the new sealer I used, and MEK (Methyl Ethyl Ketone) is what they recommended to remove the old. It is nasty stuff, but it works. The MEK along with nuts/bolts/screws or such to break it up, worked, although there was quite a bit a shaking the tank back and forth to remove it. I did several flushes of it, and all and all it took the better part of a day. Now the MEK (and other solvents) can dissolve the fiberglass itself if you let it sit too long, but I had no issues with that using it over the course of a day. BTW, I found that for me, large lag screws, with their sharp threads worked the best. Use a good quality respirator with the stuff though.
-Matt