When I worked on jets in the early '70's, they had no protective coating on the skin, even made a few replacement panels when damage occurred. There are some aluminum semi-trailers in the area ,but I don't know if any of them are tankers, but some of the others are and they are extremely shiny
. Aluminum (and I'm no expert, just years of experience with aluminum motorcycle parts) is like any surface, as you get it polished it has less and less microscopic rough areas, it's those areas that invite oxidation so after initial polishing it usually doesn't take as much to keep it up.