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?
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It means water is getting inside the body panel through some other means. The way the body panels on cars are overlapped is so they form channels for water to flow around and down to the ground. But if there’s a missing seal or gasket somewhere, the water gets inside and rusts the panels out from the inside. For example if the weatherstripping on your door windows isnt’ in good shape, water can get into the door itself. Now there should be drain vents to let the water get out of the door, but sometimes those get clogged and voila, now you have water in the door.
My car has rocker panel drain plugs in the front of the rear wheel well. Well my model year and a few years after it, the drain plugs didn’t fit quite well enough so they let water from the wheel splash get in… and the rocker panel was full of sound-deadening foam for those model years, which just soaked up the water like a sponge. Now my rocker panel is rusting out.
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.
Paint is very, very thin. There can be cracks too small for you to see which prevent the surface from being water-tight. That was one reason cars are waxed very often by enthusiasts, but if you’re not doing that then rust is a matter of time. Also, if your roads are salted, that increases the speed of the reaction by 10X.
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.
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