Some tips for cleaning a rusty gas tank include using kerosene along with metal washers to scrape off the rust particles. Also using anti rust solvents and letting them sit will allow the rust to fall off on its own.
When kerosene is poured into a rusted area, it attacks it. It breaks down the rust flakes without damaging the metal area underneath it. When used together with metal washers, the gas tank can be shaken and the hardware will actually scrape off the affected surfaces.
Anti-rusting solvents can soak the gas tank and cause the rust areas to fall off and eventually dissolve. Although easier to use than kerosene, solvents generally take longer to work.