Purchase a marine fuel system cleaner specifically designed to clean your boat's gas tank. To use these cleaners you will need to add the cleaner directly to the gas tank and run the motor to allow the cleaner to circulate through the system.
Add abrasives into the gas tank to help knock off larger particles of rust. Adding bolts to the gas tank is one method of removing larger corroded areas as the bolts will toss around inside the tank. If you are hesitant about adding bolts, add ice cubes---these will melt in the tank and you won't have to fish them out later.
Add an acidic liquid to the tank such as phosphoric acid or vinegar. When using these methods of rust removal you will need to take the boat's gas tank off and empty it so you can thoroughly clean the inside of the tank.
Use a pressure washer to knock off large pieces of rust from inside the tank. This technique will also require you to remove the gas tank and empty it so you can access the inside of the tank.
