An ominous brown stain on the fender. A bubble in the paint at the bottom of a door. Soaked floors after hitting a puddle. These are signs the iron worm has been hard at work: rust.
Otherwise fine cars are routinely sent to early graves because of rust, even though it’s a largely avoidable problem. With iron-based metals, battling oxidation can feel like a Sisyphean task. In spite of the advanced coatings and alloys developed by chemists and engineers, iron’s unstable chemical makeup means it will always succumb to rust in a natural environment. In fact, iron and most steel will completely reduce to iron oxide and other constituent elements over a long enough time.
This doesn't mean that your car is doomed! Understanding how cars rust, your car’s problem areas, and how to address that brown, flaky trouble means that your pride and joy can stay on the road as long as you do.
Rust forms as a result of the electrochemical breakdown of iron-based metals. This breakdown is the result of oxidation, which is the process where iron surface molecules react with oxygen in the environment and produce a new molecule, Fe2O3, otherwise known as iron oxide. That iron oxide is rust.
Pure iron doesn't oxidize as aggressively. Examine an old iron engine block and you'll see a thin surface layer of rust but little penetration into the metal. Unfortunately, iron alone isn't a particularly good material for building cars, so today’s cars typically use steel alloys. Adding a dollop of carbon to iron creates steel, which offers dramatic improvements in flexibility, tensile strength, and formability. However, this added strength comes at a cost, as it adds impurities that accelerate the formation of rust.
Exposed steel rusts at different rates depending on several factors: alloy components, thickness, the environment the steel lives in, and the type of heat treating the steel undergoes. Very poorly made cars in the 1970s began showing surface rust as soon as they hit the docks. Completely untreated raw sheet steel can rust through in as little as a few years.
Then there’s the effect of salt. Road salts and other contaminants dissolved in water act as electrolytes. When electrolytes are introduced to a chemical reaction, they speed up the exchange of molecular components. For drivers, this means that dirty or salty water trapped somewhere in the car's body makes that spot rust faster. It also explains why cars in northern climates where salt is used in winter as well as cars that spend a lot of time near the ocean are prone to rot.
This is why automakers do so much to try and prevent corrosion. A huge amount of testing and material science is dedicated to keeping your car from dissolving away beneath you. Aluminum and magnesium components are becoming popular not only because of their light weight, but also because they corrode at rates that are unnoticeable within a human lifetime. However, those metals are expensive and automakers use them sparingly as a result.
Alloying elements added into cars’ steel such as nickel and chromium can help stave off rust, but nothing is foolproof—everything eventually corrodes. Modern sheet steel also comes off the roll with highly durable coatings. Those are further augmented in the final assembly plants when freshly made vehicle bodies are dipped in baths of anti-corrosion agents before the painting process.
However, the road-facing side of the car turns into one big sandblasting cabinet at highway speeds, and those dips and coatings wear off over time. This is why every car owner needs to periodically inspect their car for rust regardless of where it lives or what kind of additional underbody coating it may have come with.
Rust forms in stages, and knowing where a problem spot is in that decomposition process can help point you to the right solution. Here are the three main stages of rust and how to fix them.
The first signs of a problem pop up in paint nicks, cracks, and scratches. An easy fix.
Surface rust is just that: on the surface. Leave a car with steel disc brake rotors parked for a little while, and you can see surface rust form on the rotors. That’s never a threat, however, as that rust is typically wiped off the rotor after just a few stops.
Most surface rust on your car forms when paint breaks down through mechanical or UV damage, which is why even cars in warm, dry climates get it. Structurally, surface rust is not a problem, and depending on the metal's thickness and alloy composition, a level of "passivation" may even be reached where the surface rust actually guards the metal from further oxidation.
Regardless, it's best to correct surface rust as soon as you see it—just in case. That patina-coated truck from Arizona may become a hole-filled nightmare in the wrong weather. The fix is much like general paint repair. Start by using an abrasive wheel or sandpaper (we used 50-grit) to cut through the paint and corrosion until clean, bright metal is visible. Rough up the surrounding areas, too, so that your body treatments can adhere well to the car.
Next, use a rust inhibitor or converter to impede the tin worm's progress. Your steel may look shiny after knocking the visible rust off, but there’s still some iron oxide there. Rust inhibitors convert iron oxide into a chemically stable, moisture-resistant compound that protects the rest of the metal underneath. For example, the we tried contains tannic acid, which reacts with the rust and converts it to iron tannate (which is stable) and 2-Butoxyethanol, an organic polymer that acts as a primer atop the metal. (You can paint over this Permatex version, but we recommend this one for places like the car’s underbody where you don't care how it looks as long as it's not rusting.)
Then put on some rubber gloves and apply the rust converter with a brush or sponge. You'll want to pour as much rust converter as you think you'll use into a disposable container because you can't double-dip into the bottle. If you get some rust in your container of rust converter, the compounds will start working there instead of on your car’s metal, thus ruining the contents of the bottle. Wipe a thin layer of rust converter onto the metal and wait 15 to 30 minutes for it to dry.
Then add a second layer and wait at least a half hour for it to dry. If you can keep the surface moisture-free for 24 hours, then you could prime it—this Permatex Rust Treatment isn't a primer for paint. Follow up that primer with paint and clearcoat, then buff to blend the finishes. Alternately, if it’s in a spot where people can’t see, you can just leave it alone after letting the Permatex Rust Treatment (or similar sealing rust converter) dry, secure in the knowledge that your winter beater might last a few more seasons without the bumpers falling off.
The chemical process corrupts the surface and reduces metal strength.
If you don’t correct surface rust and you allow that decomposition to penetrate further into the metal, you may see bubbles start to form in your car’s paint. This is because iron oxide molecules are physically bigger than those of iron or steel. That worsening rust issue self-propagates by expanding and flaking away the outer layers of paint and metal, exposing fresh base metal, which then corrodes.
When rust penetrates into the surface like this, it causes a rough, pitted type of damage called scale. Correcting scale means getting through the rust with a wire brush, knocking down roughness with a grinding wheel and smoothing out the surface with sandpaper. Make sure you’re down to bare metal and that there are no big flakes of rust left before sealing it back up, otherwise your sealer or filler could pop right back off. Rough up the area around the bare metal so your next sealants and treatments can stick to it. If it’s in a nondescript area such as underneath the car, you can just seal this cleaned-up spot back up with a rust converter and call it a day.
The metal surface may still be rough once you’re done sanding the larger flakes off. If it’s in a more visible spot, you may want to consider smoothing it out with a body filler such as Bondo before finishing it out with primer and paint. Follow the instructions from the manufacturer of the body filler as it can vary, but generally speaking, you mix it up to a specified ratio depending on your room temperature. NAPA recommends only mixing as much body filler as you’ll use within ten minutes as it starts to harden relatively quickly. Spread it on using a metal or plastic spreader, but leave a little extra on top to sand smooth.
It takes about 30 minutes for Bondo to fully cure and harden. After that, smooth out the extra filler you left on top so the repair matches the contours of the rest of the car. If you have a rust inhibitor that is safe to use with body filler, add this now, but otherwise, you can seal your work with a regular, high-quality primer. As with fixing surface rust, paint and clear-coat your repaired spot, then buff to blend it in with the rest of the car.
After prolonged exposure, steel is converted to brittle iron oxide and holes form.
Rust will eventually eat through your car’s metal if left unchecked. This is penetrating rust, and it’s the cause of everything from Swiss cheese-style holes in the fenders to more dangerous problems with weakened frames and suspension components.
If you live somewhere with rust-prone conditions, don’t wait for bubbles to appear on your car’s body. Take a look underneath the car periodically to inspect for any rough or compromised parts that could be a safety risk on the road if left unfixed.
As scale rust gets worse, the base metal flakes away and leaves holes. Now you’ve got a bigger problem, and you’ve got two options to fix it. You can completely replace the affected part or panel (tough), or if it’s merely a bodywork issue, you can cut the rotten parts out and weld metal “patch panels” into place (tougher). A reputable body shop should be able to take care of either issue. Welds that hold on patch panels can be smoothed out to look like they were always part of the car by the right set of talented hands.
As tempting as it may be, you shouldn’t repair these larger holes with body filler as you would with pits left by scale rust.
That being said, don’t attempt to patch your car’s frame back together. A rusted-through frame means the structural integrity of the car is questionable, and it should be inspected and repaired by a qualified repair facility. You really don’t want to be in a crash in a structurally sketchy car, so we can’t reiterate this enough: check for rust underneath your vehicle every now and then—before it’s too late.
The good news is that this kind of vehicular decay is largely preventable. The best advice is the most obvious: Wash your car regularly to keep the body and underside clean of the road grime, salts, and dirt that lead to corrosion. Many modern car washes can blast the underside of your car with water to get much of this grime off.
The not-so-obvious advice is to check the drain holes along the bottoms of doors and rocker panels, which allow rainwater to flow out. Use a pipe cleaner to clear these holes out, and keep the car’s nooks and crannies dry. WD-40 can be a useful tool here as well. This lubricant protects parts from corrosion, and its thin nozzle can be used to reach tight underbody spots and blast away water or corrosive grime.
As noted earlier, many vehicles have a thick coating on the underside that chemically seals the steel against oxidizing agents. Regular inspection and repair of the spots that have worn bare will keep rust from advancing and causing additional damage. If this coating is not there, consider adding one yourself. POR-15 is one of the more popular examples of such a rust-protective sealant, and they even have a rubberized coating to go over it in case you want even more protection underneath your car. As with other rust repairs, sand off any rust that might be sticking through and rough up the area you intend to paint with some sandpaper, then brush or spray the new coating on.
As long as your metal is properly sealed from the elements and kept clean of corrosive salt and grime, you should be able to get years of safe driving out of even the most abused winter beater.