Here is a summary of the steps I followed to restore the gas tank:

  1. Dried the tank
  2. Steam cleaned it at the car wash
  3. Dried it again
  4. Carburetor cleaner
  5. Steam cleaned at the car wash, again
  6. Dried it again
  7. Used 2 gallons of the Eastwood varnish remover, rinsed with water
  8. Added 2 gallons of 15% HCl solution, and rinsed with water
  9. Added 1 gallon of 30% HCl solution, and rinsed with water
  10. Applied carburetor cleaner again, followed by rinse with water
  11. 15% HCl, and rinse
  12. Zinc Phosphate solution
  13. Acetone rinse
  14. Sealer
  15. Dried with forced air

At each step involving the HCl, I rocked the tank back and forth for 15 minutes (per side) to clean each side. I made sure that there was some kind of an opening at all times, so that the gas generated could escape.

One of the limitations of this method is that the acid won’t dissolve the varnish and tar, and the carburetor cleaner won’t touch the rust. So I had to keep going back and forth until all traces of each contaminant was gone.