There is no glue/epoxy that will permanently repair a gas tank, as they are roto-molded or blow molded from polyethylene (nothing sticks to any ethylene plastics permanetly), the only way to repair them is buy using another piece of polyethylene cut into stips & a gas soldering iron with the blow torch attachment, to plastic weld it, you have to vee out the crack & clean around it with alcohol or acetone, then you start to melt the crack from one end, once you see the material starting to gel, introduce the strip of polyethylene & melt (knit) the 2 materials together until you close up the crack.
I've repaired many of my own tanks, but now I take them to a friend of mine's factory who makes roto-molded plastic goods & he does it for me, with a special heat gun with a very small nozzle, I hope this will help.
