The best thing I have found is the two part epoxy resin that comes with a sheet of fiberglass. It is made specificly for fuel tanks. You can even patch over a wet leak. Most places used to have it, but lately I have only found it at NAPA.
I carry this one http://www.kel-tec-cnc.com/p11.htm
I have been very happy with it. easy to carry (clips inside your waist band) and it shoots well for its size.