You can also use a method where you use klegacell, or other non reactive foam. (can't think of the type right off the top of my head, but they sell it at aircraftspruce.com)
Glue up a few pieces into a block, sand/shape it, then cover it with glass and resin. When you are done, pour some acetone in it to eat away the foam and you have a very light fuel tank that just needs a little putty on the outside for final finishing.
There are other types of resin as well, one is vinyl ester resin I believe.
Aircraft spruce specialties has an AWESOME free catalog that has a lot of good info on types of resin/foam/glues/ect ad nauseum.