There are lots of ways to clean out a rusty gas tank. Kreem makes an excellent brand of cleaner and sealer to take care of this problem. However, if you want to save money, I would recommend the following process. First, mechanically clean the inside of the tank as thoroughly as possible. You can use loose nuts and bolts along with vigorous movement to do this. Then, mix together a 10% solution of phosphoric acid (naval jelly) in hot water. The hotter the water, the more reactive the solution and the quicker the cleaning time. Leave the solution in the tank as long as necessary. It is possible to neutralize the solution, so if the rust isn´t completely gone in a few days, renew it. Immediately after you empty the tank of the acid solution, rinse it out with acetone. This will eleviate any of the residual water and prevent further rusting. I believe you can buy Kreem´s sealer by itself. Use as directed and coat the tank. This will not only seal up the inside and prevent rusting but will also seal any pinhole leaks. If you want the acid solution and the acetone as well as the Kreem liner, it comes in one single package. I believe it retails for around $35.