A car's exterior consists of the metal, (it is dipped in zinc to form a protective layer which prevents rusting), a primer paint layer, the color and then the clear coat on top. The clear coat is what stops rust and corrosion from effecting the outside of the metal. The zinc plating stop rust and corrosion from the underside of the metal. If either one is breached then rust (oxidation) will begin to eat the metal.

Since we can't do much of anything to protect the underside of the metal we can certainly protect the outside (clear coat). Clear coat is essentially plastic. About 50 microns in thickness. To give a point of reference, a post it note is about 50 microns thick. It looks really shiny and the paint (color) underneath looks deep and glossy when it is new. As soon as the car is painted oxidation goes to work.