How do cars rust underneath the paint?

Question

I have a 04 SUV, and I’m starting to notice the rust getting bigger underneath the paint. How does that happen? How do cars rust underneath the pain?

In: Other

Answer 1

It starts at some scratch/dent/hole in the paint.

Paint helps prevent the metal beneath form rusting, but once there is some kind of opening in that shield, rusting will start at the opening as water and air get into the opening and react with the metal beneath.

Then, since rust has more volume than the original metal (since there’s extra oxygen atoms mixed in now) it causes that material to expand, this popping up the surrounding paint and exposing more metal to the elements, this repeats over and over as the rust patch grows and grows.