dr_wogz and skypilot_one are right, its the water in the rubbing alcohol thats bad. Epoxy resins and most amine curing agents (hardeners) are not miscible with the water. So you get 2 phases, like oil + water. Not good for a fuel barrier, since the water phase can wind up yielding pin holes in the epoxy which let the fuel right through.

The immiscibility reveals itself as a cloudy (milky) mixture. If your epoxy is is truly miscible with whatever solvent you choose, the mixture should turn clear after being well mixed in your mixing cup.

Kurt