There are a few key terms car owners never want to hear, especially after the manufacturer’s warranty has expired. If, after evaluating your vehicle, a mechanic uses words like “failure” or “rebuild,” there’s a good chance your day is about to get very expensive. And for those of us living in northern states, the real fear comes when the topic of rust enters the conversation. This natural process can start with nothing more than a small paint chip which allows the elements to come in contact with bare metal. But rust has a voracious appetite and, left untreated, it will gradually ruin the appearance, value, and safety of your vehicle. Just as a poor diet and stress can negatively impact our quality and duration of life, rust can put cars in the grave long before their time.
Thankfully, rust is much less of an issue for newer models, as automakers have transitioned to materials and processes that prevent it from ever taking root. Brand-new vehicles are often backed by warranties (which usually last as long as the car’s basic warranty) covering surface corrosion, as well as a separate perforation or rust-through warranty that lasts longer. For example, premium brands like Audi and BMW protect against such damage for 12 years with no mileage restriction, whereas Mazda’s coverage spans five years without a mileage constraint, and Chevrolet warranties against rust holes for six years or 100,000 miles (whichever comes first). Coverage varies by manufacturer and, in some cases, model year, so check your owner’s manual or ask a dealer for corrosion warranty details specific to your vehicle.
The story for older cars is a bit different, particularly for those that have endured multiple harsh winters. For these vehicles, and their owners, rust is a fact of life. Therefore, it’s helpful to understand what causes rust, how you can work to prevent it from starting or spreading, and what to do when rust becomes an issue.
Why Does a Car Rust?
In a nutshell, rusty cars are the result of exposure.
Most vehicles are constructed of at least some steel components, and steel is made of iron. When combined with water and oxygen, iron corrodes and turns into iron oxide, otherwise known as rust. This is why you might see a spot of surface rust on a bolt holding a brand-new Ford F-150 pickup truck’s shock in place or on the roof of an old abandoned car with paint worn away by years of rain and sun. Classic car enthusiasts lovingly refer to this as “patina,” a term which signifies a weathered appearance but not rust that’s a cause for immediate concern. For a vintage car to make it so long without rusting into history, it likely has racked up most of its miles on roads devoid of ice and snow. Cars driven by those of us in occasionally inhospitable areas like New England are less fortunate.
That brings us back to the basic formula we’re discussing: rust forms when iron in steel reacts with water and oxygen. Damp air, packed with oxygen and water molecules, is enough to get the party started. We won’t take a deep dive into the chemistry here, though it is important to understand that pure water in this scenario acts as an electrolyte, or host, to iron and oxygen, which relinquish and receive electrons respectively. But, as it turns out, water is a lousy electrolyte, so this swapping of electrons is a slow process. Saltwater, on the other hand, is a far more effective electrolyte and, therefore, allows electrons to be transferred more rapidly. Put another way: saltwater accelerates the rusting process.
For drivers in areas where winter roads are regularly treated with salt, this comes as little surprise. As road salt does its job and melts snow and ice, a watery slush is created that gets into all sorts of hidden crevices that are difficult to reach with a traditional carwash or even an underbody rinse.
A car’s paint and protective clear coat, along with any applied wax or ceramic coatings, serve as a vehicle’s first line of defense. However, something as simple as a rock kicked up by another vehicle can chip the surface down to bare metal, creating a breeding ground for rust. And once it starts, it spreads. Rust will eat away at the metal to the point of being scaly. Here, flakes of rust, much like the thin rocks you might’ve skipped across a pond as a child, easily break away and fall off, though the steel underneath is still solid. When metal corrodes, it eventually loses its structural integrity and is referred to as rot.
As noted above, new cars are less prone to rust thanks to the increased use of materials like aluminum, galvanized steel, and carbon fiber. And we can’t forget about the plastic body panels affixed to early models from the now-defunct Saturn brand. Yet, except in rare cases, there remain parts made with traditional steel that can rust.
How to Spot Rust
If you’ve watched any TV show or YouTube video detailing the restoration of old trucks or cars, you’ve likely seen examples of cars that appear pristine but, in reality, are hiding some dark secrets. At least with an obviously old rusty car or truck, its flaws are in plain view for the world to see.
Regardless of what you’re looking at, a thorough visual inspection will uncover areas of concern. Some helpful tools include a magnet, a long screwdriver, and a pair of safety glasses.
If you’re not familiar with the vehicle or don’t know its complete history, start by inspecting the car body for mismatched paint and wavy panels that may signal shoddy rust repair. Also, look for cracks. Steel doesn’t typically crack, but body filler (generally referred to by the brand name Bondo) does. For vehicles that pass this test, place the magnet on various parts like doors and fenders; if the magnet doesn’t stick to a steel car, there’s cause for further investigation. In small amounts, Bondo is acceptable and even expected if any body repairs have been done in the past, but excessive use of filler suggests there are some nasty surprises below the surface. Whether those are related to rust or to other damages, they point to shoddy workmanship.
There are specific areas that rust seems to favor. In our experience, the most common is the rear outer wheel wells (the part of the body around the wheels). Debris, such as road salt, collects over time and gradually rusts from the inside out. Bubbles in the paint indicate that rust has formed and is guaranteed to spread. Be sure to check the lower section of all body panels since they get the greatest abuse from the road. Pay particular attention to the bottoms of the doors, including the tailgate or liftgate. And again, these tend to rust from the inside out, so inspect the inner edges and look for bubbling paint.
Rocker panels (the horizontal panels below the doors) are notorious for rusting, as are the lower rear cab corners of pickups. In regions such as New England, it’s not unusual to see an older Chevrolet or Ram truck with crusty metal in these spots. People looking to make or save a few bucks have been known to “fix” these problems by quickly welding replacement sheet metal directly over the rust or filling holes with spray foam which is then sanded and painted. When in doubt, pull out the long screwdriver and give the panel a poke.
Of course, rust is most at home under the vehicle. Any glaring issues should be obvious, but if the frame is scaly or the floorboards look a little soft, test them with a jab of the screwdriver. When the tip goes through what is supposed to be solid steel, you’ll know there’s a problem. Keep in mind that the brine collected during the winter can accumulate and cause havoc almost anywhere, including atop frame rails. Also, pop the hood to inspect the strut towers.
And, finally, a word of caution specifically for car shoppers. A common trick is to spray the underside of a rusty vehicle with black paint to give it a fresh appearance and mask any red flags. Some buyers may take notice of this, but there may be other quick fixes that pass under the radar. That’s why we always suggest visiting a qualified and reputable mechanic for a pre-purchase inspection, where the vehicle can be put up on a lift and examined from top to bottom. Whatever fee they charge will be small in comparison to potentially thousands spent down the road on unexpected repairs.
Fixing a Rusted Car
Deciding if or how to repair rust largely depends on the condition of the car, as well as your needs and budget.
For minor repairs and relatively little cost, body filler can be used to address a rust problem. Anyone who has Spackled a hole in their bedroom wall will be familiar with this process. Start by sanding away any rust, then apply body filler, sand again, prime, and paint. Similarly, fiberglass patches can be used, though these materials should only be used for small cosmetic repairs and never on a structural component. If you’re dealing with a rusty frame that is still structurally sound and without any holes or soft spots, it can be sanded with a wire brush or wheel and ultimately repainted, but to do this thoroughly you’ll need to separate the frame from the body. Fully boxed frames, found on many trucks, create what are essentially hollow square tubes with internal rust that can’t be reached and addressed.
When rust has devolved into rot, you may be past the point of repair. It might make more financial sense to sell the car to a scrap yard and shop for a new ride. This was a dilemma faced by participants who joined a class-action lawsuit against Toyota, claiming the frames used for certain models failed prematurely due to a manufacturing defect. In response, Toyota recalled more than one million trucks and SUVs with rot affecting the frame and critical suspension and steering components. The cost of such repairs would typically exceed what an owner is able or willing to pay.
Perhaps there is some rot, but it’s isolated to a small area of the frame or body. In this case, the affected metal could possibly be removed, but this job is not for the average do-it-yourselfer. Using a cut-out tool or angle grinder, the outer layer of metal is cut away, more often than not exposing more rust and rot on the inner panels. Much like cancer in the human body, the rust must be removed entirely, leaving only “healthy” steel to which new sheet metal may be welded. If done properly, rust should not return.
Rust Prevention
Thankfully, preventing rust is easier than repairing it.
One of the simplest and most effective methods is to keep your vehicle clean. When possible, wash not only the exterior surfaces but also the inner door jambs, and use the hose to rinse inside the wheel wells thoroughly. Always use products specifically designed for washing cars. Dish soap, for example, is often used since it’s readily available in most homes, but it can include chemicals that may harm your vehicle’s finish. Also, to get the best possible results, use separate buckets for clean and soapy water, aim to wash in cooler temperatures and out of direct sunlight, and use a microfiber towel or chamois to dry the car as soon as possible to avoid unsightly water spots.
Apply touch-up paint to any nicks that could serve as a breeding ground for rust. Better yet, dealerships and auto parts stores sell a transparent adhesive film that can be affixed to front sections of the hood, bumper, and fenders to protect against rock chips and other damage.
A vast array of products have been designed specifically for the underside of your car or truck. Most are widely available in a spray can or larger quantities applied with a spray gun. These are the modern equivalent to the Rusty Jones rustproofing available years ago, though they don’t come with a nifty sticker for your window. There are two primary types: mineral-oil-based solutions from companies such as NH Oil Undercoating, and lanolin-based offerings like Woolwax and Fluid Film. The latter are derived from sheep’s wool and are non-toxic. These products are said to penetrate the metal to prevent rust or limit its progression and must be applied annually. In our experience, independent shops charge $200-500 for this service, depending on the size of the vehicle. An old home remedy of sorts was to coat your vehicle’s undercarriage in used motor oil: this is a drippy, environmental mess best left to history.
Similarly, rust removers and converters are available. Removers include chemicals that strip rust from metal, while converters react with rust to create a paintable protective layer and prevent any additional corrosion.
Why It Matters
More than just an eyesore, rust on a vehicle can present a serious safety concern. Corrosion will eat away at body panels, the frame, and major chassis components. Eventually, this could cause a spring to snap, a brake line to fail, or one of the countless other situations that don’t end well. With that in mind, many states have strict annual vehicle inspection standards that require rust issues to be adequately addressed.
Beyond that, preventing or fixing a rust problem just makes economic sense. With the limited availability and rising prices of new cars, car shoppers are forced to consider older, higher-mileage models to meet their budget. A vehicle that easily passes state inspection without a rust problem will attract a slew of buyers prepared to pay top dollar for a clean, high-quality used car.
Or maybe you just want to ensure that your vehicle remains roadworthy for years to come. Ask any older driver in a salty-road locale, and they’ll likely share a story about a car or maybe a farm truck that was mechanically perfect and would still be running down the road if not for the rust that took it to an early junkyard grave.
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