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Used Cars: How Much Rust is Too Much?

That noted philosopher Neil Young once wrote that “rust never sleeps,” but it can certainly turn into a nightmare for drivers. Big brown rust stains and flaking paint aren’t exactly attractive design cues, and if rust gets into a vehicle’s frame or body structure, it can be downright unsafe. Although cars and trucks today are carefully engineered for crash protection, if rust causes just one component to fail on impact, it can have tragic consequences. Indeed, if a vehicle’s structure gets rusty enough, there could be a catastrophic failure even in routine daily driving.

However, that doesn’t mean that a little rust on a used car makes it a bad deal. The key is knowing how much is too much. We’ll try to unlock a few clues below, but keep in mind that the tipping point will be different for different people: Owners who are handy with a wrench may be able to tackle some of the fixes for themselves.

What Is Rust?

The reddish-brown flaky stuff that we call rust is actually a combination of iron and oxygen. Also known as iron oxide, it comes from a chemical reaction that takes place when iron (or alloys containing iron, like steel) are exposed to water and air. What happens first is that the water and carbon dioxide from the air mix together to create a weak acid, which in turn starts to dissolve the iron. At the same time, some of the water is being broken down into its separate elements, hydrogen and oxygen. As those independent oxygen atoms meet and bond with iron atoms, the result is iron oxide. Road salt and other products for melting ice can speed up this chemical reaction.

The problem? Despite iron’s impressive strength, iron oxide is exceedingly brittle. Think about how easy it is to crumble a flake of rust between your fingers, and then imagine that stuff trying to protect you and your loved ones during a crash.

Of course, automakers think about this issue as well, so most modern-day cars are engineered with extensive anti-rusting measures. These include everything from clear-coat finishes that protect both paint jobs and body panels to galvanized coatings that shield a vehicle’s steel body structure. Additionally, more and more automakers are moving away from rust-prone iron-based metals to aluminum and carbon fiber. These materials may bring certain issues to the table, but because they don’t contain iron, iron oxide won’t be one of them.

Surface Rust

Still, the concern for most customers in the pre-owned marketplace is that, over time, those rust-resistant coatings can be damaged. A stray piece of gravel or a minor fender bender is all it takes to chip a car’s paint, and any iron in the body panels will start to rust as soon as air and water reach the metal beneath it.

That means rust spots can be fairly common on pre-owned vehicles, particularly if they’ve been driven in a northern state that uses chemicals and salt for de-icing winter roads. Now, these spots aren’t necessarily deal-breakers, since they’re relatively easy to address and inexpensive to repair. The rust is simply sanded off, while the freshly exposed metal gets a new paint job and clear-coat finish to once again seal out the elements.

If the rusting process goes on too long, though, it can eat right through the metal, causing holes or allowing body panels to literally fall to pieces. This is where problems can go from merely cosmetic to dangerous, because most modern cars and SUVs are built to rely on these body panels for structural integrity and safety.

Surface rust isn’t limited to the parts of a vehicle you can see from the driveway. Exposed areas underneath a car are potential sites for rust. Again, if the rust exists only on the surface, it’s generally not a cause for alarm.

Structural Rust

More serious problems can occur when rust gets beneath the car’s surface and within its underlying components. Rust-free body panels can provide a significant boost to a vehicle’s structural integrity, but the parts doing the heavy lifting lie under the skin. Unfortunately, this area of a vehicle is prone to damage from rust-causing chemicals and water, which can accumulate there when  you drive down wet or icy roads. Further, rust only needs a tiny crack in a car’s unibody structure (or a truck’s frame) to start doing its dirty work.

Damage to a vehicle’s body structure is repaired all the time. But doing so takes a high level of expertise, and if it’s not done right, you could find yourself in vulnerable situation during an accident. With this in mind, it’s easy to recommend that most customers avoid pre-owned vehicles that show signs of structural rust. Discovering those signs is the tricky part.

For that, a CARFAX Vehicle History Report can offer some help, since it includes information regarding a vehicle’s repaired structural damage. This information is important because even if that damage was from a crash rather than rust (and even if someone has tried to fix it), any resulting cracks are potential pathways for an iron oxide invasion. To complement the CARFAX data, we also recommend getting an expert inspection that includes putting the vehicle up on a lift, for a better view of the underbody components.

The Bottom Line When Inspecting Pre-owned Cars or Trucks for Rust

If the rust is skin deep, that’s a used vehicle you can keep. But if it affects the frame, that can be a recipe for pain.

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