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

Rust occurs where moisture can reach unprotected metal.

So if you have a raw bit of metal and leave it out, it will start to rust.

With a car, an obvious cause is paint getting damaged – if it gets scratched or chipped and exposes the metal underneath, it will start to rust.

The problem is that rust can spread – all you need is a tiny pinprick or scratch at one point to let the rust take hold, but when one point rusts, that then exposes the metal directly surrounding it, so that then rusts and exposes the metal around it and so on… Because rust takes up more space than good metal, if rust gets hold under a painted area, the expansion as it rusts will cause the paint to start to bubble up.

So the bubble that you see in your paint may have started from a tiny chip or damage to the opposite (inside) face of the panel and that has started to spread.