[QUOTE=armandozx;213307]. . Tank will be sanded and gelcoated 2 layers. . . QUOTE]
Gel coat is a waste of time! It's just polyester resin with some pigment in it, so it's brittle, porous and not totally waterproof, and has nowhere near the bond strength of epoxy. And whatever you do, DON'T foam in the tank - that just traps moisture against the tank, causing crevice corrosion! Check out Dave Pascoe's article on the right way to install a tank:
http://www.yachtsurvey.com/fueltank.htm