You can go get a jug of muratic acid, $5, a jug of castrol super clean, $6, and a quart of MEK, $7 and then go to a radiator shop and get a gallon of Redicoat tank sealer $40 to $50 bucks and go through the process of degreasing it with the superclean mix say 3 cups to a gallon of hot water, wash it around, then rinse it out, then dump a quart or so of the muratic acid in to eat the rust and what is left of the galvanizing on the inside of the can. Now you drain and rinse and keep rinsing 'cause the acid will cause rust again. Allow to dry, rinse with MEK(No smoking) and then coat with redicoat. Afterruining a pair of pants and frying your hands with the degreaser, MEK, and acid because you should have bought some rubber gloves too,$5 , have one sweet Jerry can but nobody will know it because you still have to paint the outside. $68 bucks would buy a couple of nice jerry cans eh? Muratic acid will get it usable alone, it will rust again, however, I have used the above method on fuel tanks and it does work well.