[rotorwrench]

I haven't used it a model A tank yet but I still use it in a lot of other tanks and that is a lye solution. My favorite was good old Drano crystals with aluminum bits to really get a good hot reaction. It has also been discontinued since I have to substitute Black Swan crystals now. I have to mix it first and then get it in the tank without spilling it or severely burning myself. It works over night on a moderately crusted up tank. If it's real bad then it would take more to clean out the crud. It doesn't affect the corrosion though. It may also play hell with any lead that might still be in there.

In aviation we used MEK to dissolve fuel related varnish but the stuff is expensive now days. It will play hell with your lungs if you breath too much of it. Acetone will also desolve fuel varnish but it takes longer than MEK.