You only need to remove rust if the inside of ther tank is rusty. If you are lucky and the tank is not rusty, you will still want to remove any deposits from dirty, evaporated, and/or old gas. Most of us have not been lucky and the inside of the tank needed to be cleaned and rust had to be removed.
As 48bigtrucks says above, nuts or bearings or pea-stone, or small gravel, or a length of small link chain have been used by people to free up rust. Some people have used tank cleaning solvents (or thinner and/or alcohol) with whatever abrasive they have used. Be sure to rinse the tank several times after removing whatever abrasive and/or solvent you choose.
POR is one of the companies that sells
products for cleaning and sealing gas tanks (this is no recommendation for their products - read the Q&A down to the bottom - they state that sealer is not always needed).
Tim
