....leaving the entire gas cap protrusion surface nickel plated and not painted...Painting only the outside tank surface that meets up with the gas cap assembly.
Alternative is to do what you did, Eric, paint all the way up to the gasket mating surface, and cover that interface with epoxy, he says he's always used JB Weld, although I have some Loctite epoxy that's more gas impervious, I might talk him into using that. Got a coupla weeks before he's at that stage to decide.
I guess another alternative would be to do what he says, then paint a thin layer of epoxy over the paint right under (at the intersection of) the tank surface and the cap assembly. decisions, decisions... roy