Try the " Hillbilly" method for quick rust removal from a gas tank.
Simply remove the tank, and remove all of the gas, drop a dozen or so 1/4 inch nuts and a handfull of lock washers inside it, plug the fill opening with a rag, or cork, or the cap, if it isnt the guage type, and vigorously shake the tank.
The pointed corners, and edges on the nuts and washers will remove the loose rust in no time.