IMO, the best way to completely remove rust from a fuel tank is to do it by electrolysis. (https://www.youtube.com/watch?v=cTKHZSKuJtQ) It gets rid of the rust in the seams as well as the rust on the sides. Watch the you tube video. I've used it on several motorcycle tanks and they come out brilliantly. You can use it on larger tanks, too. Keep cleaning the displaced rust from the electrode every 12 hours or so (watch the video). You can also use electrolysis to remove rust from metal tools, brackets, etc.

On a badly rusted tank you may want to do several days. On one not badly rusted, one day should be enough time.

After you remove the rust you need to get rid of all the moisture left over from rinsing out the electrolyte. I've used at times acetone or methylated spirits. Dump in a quart or so in the tank and slosh it around. It will pick up the water. Then dump the acetone or methylated spirits in a responsible manner and dry the inside of the tank. I use the exhaust from my shop vac to dry my tanks. Change out or clean the filter, put in a new bag in the canister, and put the hose in the tank opening. Turn on the vac and let it run for a hour or so. Maybe 2 for a Fiero tank.)

I can say this enough, DO NOT USE A HEAT GUN TO DRY THE TANK. Also, DO NOT USE ACETONE, METHYLATED SPIRITS OR OTHER COMBUSTIBLE FLUIDS NEAR OPEN FLAMES OR ELECTRICAL EQUIPMENT THAT MAY IGNITE THE FUMES.

When the tank is dry you can reinstall it. The chances are it will rust again unless you treat it. The best thing I've found for treating rust is Red-Kote (http://damonq.com/red-kote.html). It is not affected by ethanol fuels like most other tank liners. It forms a flexible membrane that will never crack or flake. The tank does have to be completely dry for it to work properly so make sure the tank is dry before using it.

Also, an extra benefit is that Red-Kote will seal pinholes. I use two or three coats depending on the condition of the tank with at least 12-hours between coats. The Red-Kote that drains out after coating the tank can reused. I position the can under the tank opening and let it drain back into the Red-Kote can for an hour or so before resealing the Red-Kote can.

Good luck.