The best and easiest way to clean the inside of a rusty gas tank is to drain it, un-mount it, swish about a quart of lacquer cleaner around in it, drain it again, let it air dry for a day or so, put two to three quarts of clean 1/4" to 3/8" crushed (squareish sharp edged) gravel into the tank, put the cap back on, strap tank securely to the outside of a tractor rear wheel, drive a round for a couple of hours with the rocks tumbling all around inside the tank.
You have to drive around at a slow enough speed so that you can hear the gravel 'tumbling' around inside the tank. If you drive too fast, the centrifugal force just keeps the gravel pined in one place and no 'scouring' will take place. Check the progress with a flash light and repeat as necessary. When all clean, wash out with detergent and water - then wash with alcohol and blow dry with a shop vac blowing into the tank. Lightly oil the inside with some kerosene and drain. -- Good as new !!
Hope that this helps, NJDale