Any chemistry majors? I have a technical problem that is driving me bonkers. The gas tank of my 1972 Vespa in encrusted on the inside with a dark brown scaly deposit that doesn't want to come off. Working on it through a hole about 4 cm in diameter limits the removal methods I can use. I need chemical action. The trouble is I don't know what it is. Some of it may be rust, but most of it was residue left by very old gasoline mixed 50 to one with oil. Aren't both gas and oil hydrocarbons? What is there that will dissolve compounds of hydrogen, carbon, iron, and oxygen, without dissolving the gas tank? A reproduction gas tank is not available for this classic old scooter, and if was, it would be rather expensive.

I've tried Gunk, brake cleaner, carburetor cleaner, and even rattled a hand full of nuts and bolts around in there. I don't want to put water into a fuel tank, and a blow torch might present problems.

Any suggestions?
Howard