There's another method, using electrolysis to remove rust .
Quick explanation, plug the hole(s) for the fuel tap .
Find, or make a bare metal spot on the under side of the tank, so a battery charger clip can be secured to it (negative).
Mix up a half to one cup of washing soda (calcium carbonate) in water to get it dissolved put it in the tank fill the tank with water to within an inch or so from the top .
Make an insulated holder for a piece of wire with a piece of steel or iron to hang down inside the tank .
Place the positive lead from your battery charger to the wire at the top of the tank, turn the charger on .
I think 1 amp is sufficient .
The rust will be deposited on the piece of metal hanging in the tank .
I think they recommend a piece of metal with a large surface area .
Check it every day or so, remove and clean it if it gets covered with rust .
I've also used vinegar, it takes about 2-3 weeks for this method to work .