As any former owner of a Chevy Vega can tell you, galvanic corrosion is a BIG problem. The Vega had a steel hatch, but an aluminum frame around the rear window. Despite the fact that the hatch was painted before assembly, over time the inevitable would happen, and the hatch would rust through all around the aluminum frame. (Being a basic idiot at the time (and still), I repaired mine by filling the numerous holes with exterior wood putty!) So beware the fuel door, even if the paint is sound, without some sort of buffer gasket. As to the bike issue, I think the corrosion is greatly helped along by the presence of an electrical current. Cars' bodies carry the power ground, bike frames usually are not powered, so that could account for the difference. But I defer to the engineers in the group on this one.
