Here's what I used:
Kreem Products Fuel Tank Liner Combo-Pak 1210 https://amzn.to/2UBWvUu
White House Apple Cider Vinegar https://amzn.to/39oc4Dd

Motorcycle gas tank filled with rust cleaned to bare metal with one of these two solvents. Apple cider vinegar versus Kreem fuel tank liner. We test to see which one is worth the money. Do you want to save money or time? You can't choose both, so let us show you which method we chose and why. You will be astonished at the results