How Does Rust Spread?

In the automotive restoration world we refer to rust like it is a “cancer” or infection that spreads in metal. So we often get questions on how and why rust spreads. Rust is iron oxidization that occurs when bare metal that contains iron is exposed to moisture and oxygen. As a rule of thumb, if oxygen and water/moisture can’t get to the surface it is safe from rust.

So how does rust spread and why does it spread like it does on certain objects versus others? That question is a really tricky one and can be answered identically on a case by case basis. Rust ALWAYS starts when bare metal is exposed to oxygen and water. On most vehicles it is a design flaw in which a part is constantly exposed to water or corrosive materials and it breaks down the factory paint or coating. Once the coating is compromised moisture and oxygen can get in and start the rust process. This means even something as small as a rip in seam sealer or a water drain being clogged in a car door can expedite the rusting process.

A common misconception is that rust is like an infection that will spread on its own. The science tells us otherwise. The metal needs to be exposed to water and oxygen, so you can have an area that is extremely rusty and an inch next to it is completely fine because the paint or coating wasn’t sacrificed. Now often “rust creep” occurs when the rust gets under the coating and can work out from the center of the compromised coating. If you let coating deteriorate over time it can “creep” but it is the failing of the coating that allows the rust to spread.

If you want to correct or stop rust “creep” you need to remove the rust or seal it up so water and oxygen can’t get to it and allow it to work. Small rust bubbles or pinholes are usually the tip of the iceberg and you need to remove some of the coating around the bubble to fully expose the rust and remove the failing coating. Learn how to spot repair rust bubbles HERE.

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