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Underbody rust explained: When to be concerned, what to do about it

2021 F-150 rust

Another look at the rust on Tim Esterdahl’s 2021 F-150. (Photo by Tim Esterdahl)

Recently, there was a bit of a hullabaloo about brand new 2021 Ford F-150s having underbody rust. A forum dedicated to the 14th-generation pickup truck kicked it off with a simple question: Rust on rear end of 2021 F-150 — any others?

Come to find out, there were a lot of others — including our publisher’s own truck. So, we jumped into the fray talking about rust issues on this brand-new truck.

Understandably, after we posted our story and video about the F-150 rust issues, we got a lot of comments, questions, criticisms and accusations of idiocy. So, we thought it was worth digging into this issue a little deeper and asking: Is rust on your underbody normal, or is it a problem?

Regrettably we couldn’t get any additional comments from Ford, and frankly, a lot of industry folks we reached out to declined to talk to us on the record.

However, through several conversations with engineers and industry experts, we were able to come up with a little more clarity. And, for the most part, Ford’s comment that this is a cosmetic issue and poses no impact on part performance or life is true. Ish.

So, let’s dig into some of the questions we’ve been fielding.

Is rust on the underbody of a new vehicle normal?

The answer we’ve come up with is: Kind of. Once upon a time, most automakers painted the underbody, so it looked shiny and new at the purchase point – and probably for a few years beyond. Similar to Thanos in “Avengers: Infinity War,” however, rust is inevitable. Once that paint chips, the rust will creep in. It just may not happen immediately.

Due to cost concerns, most automakers have stopped painting undercarriages because it saves money, and it’s not really that big of a deal. I mean, who other than our publisher (and now me — insert face palm) regularly crawls around under their vehicles? So, why pretty-up something most people never see? Especially when it mostly won’t cause a problem.

Well, as Eric Mayne, media relations manager for vehicle safety/quality/regulatory compliance at Stellantis, points out: It’s a customer satisfaction issue.

The automakers we’ve discovered so far who still coat or paint the underbody? Toyota Motor Sales and Stellantis (formerly, Fiat Chrysler Automobiles).

Mayne confirmed the automaker uses a process called eCoating (aka electro coating) for the underbody parts, which basically coats everything from the frame to the drive shaft in a resin polymer solution.

underbody rust

A recent 2021 Jeep Wrangler test vehicle with the Stellantis eCoating shows no sign of surface rust after more than 1,400 miles in the midwest. (Photo by Jill Ciminillo)

Is this rust concern? Why or why not? 

Not really. Ish. We put the “ish” in there again because when you’re talking about something like U-joints, the rust actually serves as a protective coating.

Where you should be concerned is when something hollow, like a hanger for the muffler or the pop converter, is rusting. In something like that, the rust could eat all the way through creating a hole. When something like that happens, that’s when your muffler might come loose and drag on the ground while driving. Does that affect the performance of the part? Well, that depends on your definition of performance.

At the end of the day, our sources called this more of a customer dissatisfier than a true problem.

What elements might contribute to this rusting?

There are a lot of contributing factors to the formation of rust – like if you live in a place where roads are salted in the winter or if your climate is humid. Yes, yes, that’s over time – so what about these new trucks with rust? Again, no one would comment on the record, but one theory we heard was this could have been a supplier issue, and that during the Covid-19 shutdowns, the parts were stored improperly.

So, if you have parts that don’t receive a protective coating and then are exposed to harsh climates, a lot of mud or corrosive materials, the result is rust.

Is there anything that can be done to prevent rusting?

If an automaker isn’t painting or coating the parts on the under-side of a vehicle, it might be worth taking matters into your own hands. There are companies out there like Ziebart that will provide professional underbody protection.

In fact, in a recent video, we featured a company called New Hampshire Oil Undercoating (NHOU), which coats the underbody of vehicles with oil to protect against rusting.

If you’re a DIY-er and think you can manage to take care of the coating by yourself, you can use something like 3M Dynatron Dyna-Pro paintable rubberized undercoating or even Rust-Oleum. NHOU even offers a take-home DIY kit. But here’s the thing, your underbody has to be pristine before starting, and you have to make sure you don’t miss anything – or defeats the purpose and lets in moisture and debris anyway.

But the biggest thing you can do to prevent rusting is to keep the underbody clean – especially after driving on salted roads or taking your vehicle off-road in a muddy location.

So, you know that undercarriage wash? Not a scam.

What should you do (or not do!) if you see this rust? 

After we posted our story, it prompted a lot of owners to crawl under their own vehicles (F-150s and otherwise) and express dismay over the amount of rust under their vehicle. Which then lead to proclamations of getting rid of said rust by any means necessary. We heard mentions of sandpaper, high-pressure washing and several other methods to dispose of rust, and we want to say: STOP!

If you see rust and are concerned, seek professional advice first. As we mentioned above, some rust serves as a protective coating and removing it could actually damage your vehicle.

But if you’re truly concerned, Joey Dupont, NHOU shop manager, says his shop’s oil undercoating can help stop the rusting process once it’s begun. While it won’t kill the rust, the coating will cut off the oxygen needed for rust to continue.

The bottom line on underbody rust

While nobody wants to see rust on a brand-new vehicle with just 500 miles on it, rust on a muffler is very different from rust on the frame. Yes, it looks bad. No, it probably shouldn’t be happening. But it’s probably not the end of the world. You should have it checked. And you should also consider adding a protective coating after having it professionally cleaned.

Yes, we were ultimately dissatisfied with Ford’s only on-record comment to us: “While some F-150 underbody steel components may show signs of surface rust, this will have no impact on part performance or life.”

But unless you’re seeing rust on the frame, you should probably just leave it alone.

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Jill Ciminillo

Jill Ciminillo is a syndicated automotive writer. Jill also manages the “Drive, She Said” blog for ChicagoNow and posts reviews to DriveChicago. She is the president emeritus of the Midwest Automotive Media Association and has the distinction of being the first female president for that organization. She also serves as a judge for the Automotive Heritage Foundation Journalism Awards. Previously, Jill has been the automotive editor for the Sinclair Broadcast Group, the Chicago Sun-Times News Group and Pioneer Press Newspapers.

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