Planes do not have to be painted. Bare aluminum is pretty corrosion resistant. Many airlines strip most of the paint to save weight.
"While the lighter weight of a polished airplane saves fuel costs [...] this savings is more than offset by the higher cost of washing, polishing, and painting a polished fuselage throughout its service life. The net operating cost of polished airplanes, calculated as a percentage of the total operating cost, is between 0.06 percent and 0.30 percent more than the total operating cost of fully painted airplanes."
http://www.boeing.com/commercial/aeromagazine/aero_05/texton...
https://en.wikipedia.org/wiki/2024_aluminium_alloy
Some corrosion pics still at http://connielinguscorrosion.blogspot.com/ .