Originally posted by Parallax Most people don't realize that aluminum oxidizes; and it does so quite rapidly. It is a very thin layer, and discolors the metal very little so is nearly invisible. Except under extreme conditions it remains superficial and does no harm but it really interferes with paint being able to stick.
The aluminum used for the exterior of airplane skins is ALCLAD, which is an aluminum/copper/manganese/magnesium alloy coated on the exterior side with a very thin layer of pure aluminum. This is supposed to reduce the amount of corrosion.
Still, it requires care and maintenance to maintain the structural integrity of the exterior surface of the plane.
One airline, American Airlines, has a mostly bare and polished finish on their planes. I suppose that keeping it polished and shiny is no more labor intensive than washing a fully painted airplane.