Hey Doug,
Does you fiberglass tank have a liner or interior coating on it? If so, do you know what it is?
If not, do you know what resin was used in the fiberglass composition?
Cheers,
Most commercial tanks use gell coat as a sealer, Gell coat is fiberglass resin mixed with talc, with or without pigment for color
Use Epoxy resin (available at west marine and many other places, WM is not the cheapest) thin with whatever is recommended, pour into the tank, and work over the interior the same way as you'd apply tank sealer.
Same method's used in building boat tanks.
Alternate, build tank with a styrofoam plug, with a good layer of resin over it, layup the fiberglass then use gasoline to dissolve the styrofoam out of the tank leaving the inner gellcoat.
if you buy gellcoat in a can, it's normally poly
Recommend Epoxy, there have been several statements made that modern gas formulations (primarily alchohol blends) will break down regular poly based fiberglass resin.