Quote Originally Posted by Tinker1980 View Post
A few words on steel and coatings, from someone who used to deal with both...

Our little Mitsubishi Mirages start out as various raw materials. One of those materials is a big, ~40,000 pound roll ("coil") of steel sheet. That sheet is rolled, broke, formed, punched, welded, and pressed until it becomes a unibody for a Mitsubishi Mirage. Once we have this perfectly formed, lovely, virginal Mirage Body, we're going to dip it in a saline/zinc solution, drop a zinc anode in there, and turn on the current. Once our body comes out of that process, it's galvanized. You could throw it outside at this point, and it wouldn't rust for decades, unless it got a good scratch or you showered it with salt water every day.

AFTER that rust-proofing process, there are various steps where the body is dipped in primers, corrosion inhibitors, and finally, it's painted. The paint, as CobraJet pointed out, is just a thin covering on the top. The important parts have already been done, before that car body knows what color it's going to be.
This is a cool theory. Now come to the upper midwest and see tons of late model cars i.e. 5 or 6 years old even with rust popping out from under the paint. I wash my cars a lot so my Hyundai did not have any on the body at 8.5yrs but boy oh boy was it taking it's toll on the underbody.