Get a couple gallons of metal rescue, fill your tank to the top and let it sit over 24hrs, then empty out and clean with degreaser ( usually one is recommended by your coating kit or included) and at this point remove petcock and fuel senders and put tape on the outside covering those holes, then coat the tank and you are moving the tank around slowly allowing the coating to cover the entire surface. Let it dry slowly and cure, reassemble and put it back on and you should get many more years out of your tank.