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Car Rust: How Much Is Too Much? (and How to Fix It)

Car Rust How Much Is Too Much
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Why Cars Rust and What You Can Do About It

As cars age, one of their biggest enemies is rust. A little rust on a car’s body is fairly common, and it isn’t a big deal if caught early and fixed. However, if it gets into a vehicle’s frame, it can become a safety issue. Today’s cars and trucks are carefully engineered to survive crashes, but if rust compromises a vehicle’s structure, there could be a catastrophic failure in even routine driving. The key is knowing how much rust is too much.

What Is Rust?

Rust is the combination of iron and oxygen, also known as iron oxide. It’s created as the result of the chemical reaction when iron (or alloys containing iron, like steel) is exposed to water and air over time.

First, moisture and carbon dioxide in the air mix to create a weak acid that starts to dissolve iron. At the same time, some of the water is being broken down into its separate elements – hydrogen and oxygen. As those oxygen atoms meet and bond with iron atoms, the result is iron oxide.

Road salt and other products used to melt ice can speed up this chemical reaction on a car’s undercarriage or in other exposed areas.

Why Is Rust Bad for Your Car?

Why is rust a danger? Because, 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 car crash.

Most modern-day cars are engineered with extensive anti-rusting measures, including clear-coat paint finishes that protect both the paint and body panels, as well as galvanized coatings that shield a vehicle’s steel body structure.

Additionally, automakers are moving away from rust-prone iron-based metals to more rust-resistant surfaces such as aluminum and carbon fiber. These materials may have their own issues, but because they don’t contain iron, rust won’t be one of them.

Close-up of surface rust on Scratched Car Door.
Photo Credit: kmatija / Getty

Surface Rust

A stray piece of gravel or a minor fender bender can chip a car’s paint, exposing the bare metal beneath. Any iron in the body panels will start to rust as soon as air and water reach it.

Because of that, rust spots are common on used cars, particularly if they’ve been driven in a northern U.S. state where chemicals and salt are used to de-ice winter roads.

Now, these spots shouldn’t necessarily be deal-breakers, since they’re relatively inexpensive to repair. The rust can simply be sanded off, painted, and given a clear-coat finish to seal out the elements.

But if the rusting process has gone on too long, it reaches the stage at which it begins to flake. This is called scale rust, and if it continues, this penetrating rust will eat right through the metal, creating holes and leaving body panels to fall to pieces. At that point, problems go from cosmetic to dangerous, because cars rely on these body panels for their structural integrity.

Surface rust isn’t limited to the parts of a vehicle you can see. Exposed areas underneath a car are potential sites for rust.

Inspecting under a rusty car
Photo Credit: GCShutter / Getty

Structural Rust

The most serious problems occur when rust gets beneath the car’s surface and within its underlying components – those parts doing the heavy lifting lie under the car’s skin. Unfortunately, this area is a prime location for rust. Rust only needs a tiny crack in a car’s frame to begin its work.

If there’s been damage to a car’s body structure, make sure it’s repaired correctly. If not, you could find yourself in a vulnerable situation during an accident. Most customers should avoid used vehicles that show strong signs of structural rust, but discovering those signs can be tricky.

For that, a CARFAX Report can offer some help – it has information on any repaired structural damage or accident history. This is important because any cracks are potential pathways for rust. Carfax recommends getting a trusted mechanic to look over any used car, because once the car’s on a lift, your mechanic will be able to see things you can’t.

Should You Buy a Car With Rust?

If the rust is only skin deep, that’s a used vehicle you can keep. But if it affects the frame, that can be a recipe for pain. Check the vehicle history, and have a mechanic check out the areas you can’t.

DIY: How Do You Fix Car Rust?

You can apply touch-up paint to stone chips, small scratches, and other minor nicks and dings, but truly repairing rust takes several steps, a variety of tools and materials, and quite a bit of skill.

If you’re skilled enough, you might be able to tackle surface rust and small rust spots.

  1. First, use fine-grit sandpaper or a razor blade to remove the rust.
  2. Treat the area with a rust inhibitor and apply primer.
  3. Cover the area with touch-up paint that matches the color of your vehicle. It’s available from dealers, auto parts stores, or automotive paint suppliers.
  4. Depending on the size and severity of the rust, blending the repaired area with the surrounding paint may require wet sanding or buffing. Wet sanding involves mixing an automotive soap with water, soaking fine-grit sandpaper in the solution for several minutes, and then gently sanding the repaired area to blend it in. This is a process that requires skill and can do more harm than good if it’s done incorrectly.

If there are larger areas that have been eaten away by rust, that kind of repair is best left to professionals.

5 Ways to Prevent Car Rust

man washing car
Photo Credit: Getty

There are steps you can take to prevent rust from forming on your car. From a simple wash and wax to paint touchups and leak checks, here are 5 simple steps that will minimize your car’s rust risk:

1. Wash and Wax Your Car Regularly

The best way to prevent rust is the easiest: Wash and wax your car often.

Dirt can retain and trap moisture, and road salt, bird droppings, and other corrosive materials will eat away at paint if they’re left unattended.

Washing your car frequently will remove these corrosive materials. Waxing it on a regular basis (twice or more each year) will add a protective surface to the paint and clear coat. Wash and wax more frequently if you live near an ocean or in an area where highway crews spread salt on the roads to melt snow and ice during the winter. Salt is Public Enemy No. 1 when it comes to rust prevention.

2. Wash the Underside of Your Car

Many car washes offer an underbody spray that can help remove salt, dirt, and grime from critical areas you can’t easily see, such as the frame, suspension components, and wheel wells. These are all prime breeding grounds for rust.

3. Keep Your Car’s Drain Holes Clear

The drain holes are located on the bottoms of doors and on the rocker panels (the area below the doors); keeping them won’t let water accumulate and cause rust. This problem is particularly acute on minivans and large vans. Use a coat hanger or pipe cleaner to make sure the holes remain open.

4. Check for Water Leaks

Check the fender liners and other areas under the hood, along the sides of the engine bay, for any standing water. Check the trunk or cargo area to make sure water isn’t seeping past the seals.

5. Stay on Top of Paint Chips and Dings

Stone chips and other nicks and dings that are left unrepaired often develop into rust spots over time, so it pays to buy some matching paint to cover these imperfections.

Next Steps

If you have questions about this story, please contact us at Editors@carfax.com