I restore vintage bikes, so I'm restoring rusty gas tanks all the time.
The best rust remover I have ever found is "Evaporust". I buy mine at O-Reillys Auto Parts because they stock the gallon size. (It's cheaper by the gallon.) It removes rust better than anything else I've ever tried and it's completely environmentally safe and safe to get on your hands.
When you get the rust removed, you're NOT DONE. The tank will flash-rust within a very short time after rust removal. (In minutes, not hours).
You have to SEAL the tank immediately after rust removal. The best sealer/tank restoration kit I've ever found is "POR-15 Motorcycle Fuel Tank Repair Kit". Here:
http://www.por15.com/POR-15-Motorcycle-Fuel-Tank-Repair-Kit-_p_106.html
I've used others, including Red-Kote, but the POR-15 Kit is the best. I've done over a dozen gas tanks with it. The POR-15 kit comes with its own rust remover called metal prep. That will be enough for a tank with just a bit of rust in it. But, if your tank is completely rusted inside top to bottom with heavy rust, your gonna need some Evaporust first before you use the POR-15 kit.
Do it right the first time, or you'll be doing it again soon after.
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