You will receive many replies. If you are fairly confident there are no pin-holes, use Prep N Etch. Available at Home Depot. Some will say use vinegar - which works, but you have to be very, very careful of flash rust after you empty the tank. I have made tanks worse using vinegar. But then again, that's just me.
Prep N Etch should still work, but I'll let those with more experience in this help. I know there are ways to remove failing tank liners, but that doesn't sound like what you're looking for either. Sorry, I'm not much help here.
Muriatic acid (Home Depot paint department..you can get the stronger stuff in the pool area).. make sure you use gloves and glasses (safety) and lots of baking soda to nuetralize the acid when your done..
A qick wash should be sufficient to remove the "varnish"...
Full strength, moved around to contact all surfaces (works pretty fast). Dumped out rinsed tank with water (multiple times).. then some gas with some oil added to coat surfaces (keep from flash dust)...
The acid used and removed needs to be neutralized (add baking soda SLOWLY) untill it stops "reacting"..
This is probably a bit late to answer your question but may be helpful to others in the future. The old fuel residue should be soluble in lacquer thinner or acetone. If there is rust in the tank after removal of the varnish the best answer is Evapo-Rust. It is totally non toxic and water soluble. It can be used for all forms of rust removal. For a tank you really should use enough to fill the tank. The stuff can be reused numerous times so you won't waste it by fill the tank. It is truly amazing stuff.
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