The Detroit Post
Saturday, 16 October, 2021

How Much Undercarriage Rust Is Normal

Maria Johnson
• Monday, 12 October, 2020
• 7 min read

Robert Robert2111 gold badge11 silver badge33 bronze badges By the time the inspection was completed (and failed), I had a hole in my muffler system along with about $2,500 worth of rust related repairs for a $1000 vehicle.

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However, to check you're going to have to do the squeeze test between fingers and thumb on the thicker structural sections, then if you still can't determine how severe or not the rust is. It's time to get an old screwdriver out and tap (with the handle part) on the rustiest bits. If the material is corroded right through holes will likely appear, this is not good especially in the structural sections, if this is the case then you may like to walk away from this one as welding will be required.

If however it is just surface rust then a scrap off and clean up followed by an application of under body protection will suffice and your good to go. The seller should have no issues at all with you taping on the structural parts of the body to check its integrity.

In some parts of the world where lots of salt is used in the winter, this is not unusual rust. If you want to keep it longer term, I'd treat the rusty areas on the under body with a wire wheel brush on an angle grinder, rust converter and fresh undercoating after a thorough clean.

The underside of your car is at a higher risk of corrosion, because it’s more likely to stay wet after you drive in a rainstorm, as the sun can’t reach it to dry it out. Over time, as these areas stay almost constantly wet, that trapped water will wreak havoc on the metal of your vehicle.

Salt speeds up the electrolytic reaction that occurs between iron and oxygen in the presence of water, so a wet and salty undercarriage during the winter months is at an even higher risk of rusting out. You can help keep things free of corrosion by adding your own light covering of oil or undercoating, which will cling to the metal and repel water.

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Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time. I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.

If rust gets into a vehicle’s frame or body structure, it can become a safety issue for drivers. In fact, if a vehicle’s structure gets rusty enough, there could be a catastrophic failure even in routine daily driving.

First, moisture and carbon dioxide in the air mix to create a weak acid that starts to dissolve the iron. 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.

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 used vehicles, particularly if they’ve been driven in a northern U.S. state that uses chemicals and salt to device winter roads.

But if the rusting process goes on too long, it can eat right through the metal, causing holes and allowing body panels to fall to pieces. This is where problems go from cosmetic to dangerous, because modern cars and SUVs rely on these body panels for their structural integrity.

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The most serious problems occur when rust gets beneath the car’s surface and within its underlying components. Rust -free body panels boost a vehicle’s structural integrity, but the parts doing the heavy lifting lie under the car’s skin.

Unfortunately, this area of a vehicle is often susceptible to rust -causing chemicals and water, which can accumulate there when a car drives down wet or icy roads. Most customers should avoid used vehicles that show strong signs of structural rust.

CARFAX also recommends getting an expert inspection that includes putting the vehicle up on a lift, to give your mechanic a better view of under body components. You can apply touch-up paint to stone chips, small scratches, and other minor nicks and dings, but truly repairing rust can take several steps, a variety of tools and materials, and quite a bit of skill.

Depending on the size and severity of the rust, blending the repaired area with the surrounding paint may require wet sanding or buffing the surface. 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 on metal surfaces for long periods.

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.

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Also, frequently 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.

Stone chips and other nicks and dings that are left unrepaired can develop into rust spots over time, so it pays to buy some matching touch-paint to cover those imperfections. My husband’s parents recently gave us their 2002 Lexus SUV.

They had their mechanic (near a Northern city which is one of the snowiest in the US) check out the SUV, and he said it was in good shape. My mechanic here in the Mid-Atlantic checked it out and said the undercarriage, fuel lines, and brakes were quite rusty.

With 2 young children in daycare, we’d prefer not to have the additional burden of a car payment at this time. I have a 2002 Toyota Tundra that has a recall notice relating to the problem with many of them having a rusty undercarriage.

In my experience the body goes long before the structural components, IMHO (just my humble opinion) Find a shop who will put the vehicle up on a lift and let YOU take a look at the undercarriage … Even if you are not an automotive expert, you will be able to judge MUCH BETTER than we can just how extensive the rust is… If there is a mechanic there with you, ask him if he sees anything “Critical”… Pay close attention to the brake lines.

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Have the mechanic point them out… If the brake lines are badly rusted, they can be replaced at moderate expense. People living in my area (near Buffalo NY) develop an eye for what is critical rust and what is not but it's hard to describe.

Many of the critical parts subject to rust are front suspension and steering components and if all is OK there then the vehicle is likely still solid and good to go. If you’re paying someone do do these repairs, don’t be surprised if you reach a point where you’re asking yourself if it’s worth keeping the vehicle.

So how do you check to make sure your prospective car isn’t saddled with rust problems? And it’s not just the structure; rust can corrode various parts, rendering them useless unless completely replaced.

Unfortunately, rust issues aren’t confined to cars from one certain manufacturer or age group. Rust problems are more common in humid climates and in areas where road crews use salt to keep ice off the streets during the winter.

Areas such as the Upper Midwest and parts of the Northeast are especially known for rusting vehicles, largely because they suffer from both humidity and heavy road-salt use. But just because you’re located in an area that isn’t known for humidity or salt use doesn’t mean you’re safe from rust.

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If you have concerns about rust, allow the seller to let you take the car to a mechanic, who can put it up on a lift. Common rust spots include the frame rails, which run underneath a car’s doors on each side, the wheel wells, the exhaust, the suspension and virtually any other underside components made of steel or metal.

Once you have the car back on the ground, pull up the trunk carpeting and check for any signs of rust. While we generally suggest consulting a professional about these issues, you can usually remove rust spots by sanding them away, so they don’t become worse and create further problems with your car.

When road departments use salt to melt the freeze, that mineral can wreak havoc on your vehicle. In those parts of the country with freezing winter temperatures, drivers know that warming the cars up in the morning isn't the only inconvenience.

A salt and sand mixture is frequently spread over roads before or after a snow or ice storm. The sand helps keep the salt in place, plus it adds a bit of traction to wet and often slushy roads.

It can cause major body and undercarriage damage to your vehicles unless you take extra care and precaution. If you're one of the many who must travel the saline streets in the land of the ice and snow, we have some great tips to help protect your vehicle from the ravages of road salt.


Road salt, while helpful for safer driving, can cause rust and corrosion on your vehicle. Since your vehicle's undercarriage is completely exposed, this is the area most at risk from deterioration from road salt.

Keep in mind that rust on essential parts, from the axel to the brake system, can be very dangerous. If you live in an area where salt is used on the roads regularly, the risks of vehicle damage are much higher and should be taken seriously to protect your investment as well as your safety.

This coating will help prevent salt and water from the road sticking to your vehicle's metal parts. Get a pre-winter inspection: A quick trip to your mechanic for a once-over is a great way to go into winter with a safe and fully-functional vehicle.

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