Does Undercoating Stop Rust?

Modern rust proofing offers more than one way to protect your car.

Benjamin Hunting | 

May 5, 2022 | 3 min read

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Despite major improvements in automotive construction and materials preparation, rust remains a fact of life for motorists who live in cold climates, where road salt is a regular feature on winter roads. Fortunately, an entire industry has sprung up around preventing corrosion and protecting vehicles old and new from the ravages of rust.

But do these modern protective techniques actually prevent rust? Here’s a look at how rust-proofing methods work to protect an automobile.

Installing a Salt Shield

Sometimes the words “undercoating” and “rust-proofing” are used interchangeably, but really there are multiple methods used to add a protective layer between a vehicle’s vulnerable metal and the salt and moisture that lead to corrosion.

“Undercoating” typically refers to the practice of spraying a substance, usually petroleum-based, under a vehicle’s bodywork to coat the chassis and suspension. The advantages of using an oil-like or tar-based undercoating are that the coating can penetrate into small gaps and crevices—the same spots where salt, brine, and water tend to collect—with minimum effort. The oil- or tar-based substance also can soak through any existing surface rust to sit on the metal below, and it often continues to seep several additional inches after it has been applied as gravity pulls it down into channels and seams in the steel.

Where can you get an undercoating sprayed? Many independent mechanics provide this service during the fall season, and most new- and used-car dealerships also offer it. Prices vary depending on where the service is performed and the type of vehicle. Leading rust-proofing brand Krown, for example, charges about $140 for a car the size of a Honda Civic and about $160 for an SUV the size of a Cadillac Escalade.

Comprehensive Coverage

Not all oil-based rust-proofing techniques are restricted to a car’s underside, however. It’s also possible to target other vulnerable body panels and components with the same type of coating and achieve similar levels of protection. This is the model for companies like Krown and Ziebart, both of which operate out of franchised, standalone locations. During a service appointment, the products are applied to the interior of a vehicle’s tailgate, doors, rocker panels, and other areas.

This is a more involved operation than a salt shield, and occasionally involves light disassembly or the drilling of tiny access holes for spray equipment, which means the price tag is higher than a simple undercoating. Still, the resulting protection is much closer to comprehensive, and often necessary for climates where excessive levels of road salt are a fact of life.

Solid Barrier Options

There are other types of undercoating that are typically offered when a vehicle is brand new. Some dealers offer a wax or rubberized undercoating that provides a solid barrier between salt and metal—this has advantages and disadvantages. A solid covering can only be applied on a brand-new model (to prevent accidentally covering existing rust or grime). Additionally, any scrapes, nicks, or cuts that occur over time in a hard covering can let in moisture and salt, trapping them against the chassis and potentially accelerating rust, whereas an oil-based undercoating flows into any abrasions that might result from normal driving. For this reason, oil-based undercoating is often suggested as a secondary step even after the application of a solid barrier.

Rust-proofing jobs—particularly the non-rubberized type—often are performed annually to slow or prevent rust damage. Companies like Ziebart offer warranties for their rust-prevention services. Although it’s impossible to eliminate corrosion entirely, taking precautions can extend your vehicle’s time without rust.

Benjamin Hunting

Benjamin Hunting is a writer and podcast host who contributes to a number of newspapers, automotive magazines, and online publications. More than a decade into his career, he enjoys keeping the shiny side up during track days and always has one too many classic vehicle projects partially disassembled in his garage at any given time. Remember, if it's not leaking, it's probably empty.