Corner Wrench: Undercoating — miracle or mess?

Jul 09, 2023

Know the difference between rustproofing and undercoating treatments

We talk lots on Driving about the importance of rust-proofing products, but what about undercoating treatments for your vehicle? Though they sound similar, there is a difference. Undercoating sprays can provide some level of corrosion protection, but they’re mainly designed to deaden road noise and protect sensitive undercarriage areas from damage. So why bother?

Carmakers have had their products on diets for years in attempts to meet government-mandated fuel economy ratings. A great way to strip weight out of a vehicle is to get thin on insulating materials like carpets and underlays, or by deleting any undercarriage spray treatments. This lightening applies to sheet-metal as well; all of those stamped ridges in the metal you see under modern autos exist to bring back some strength lost by thinning panel steel. Hit a rock or a piece of road debris the wrong way, then, and you’ve got a good-sized dent or a puncture. With this in mind, bulking up with some undercoating for insulation can make for a quieter ride with a little more impact protection.

DIY spray undercoating is one of the few jobs that can actually be done better at home than in a pro shop. Watch a tech working in a rustproof/undercoating facility and you’ll see them using a wide nozzle spray gun that covers everything in its path. The problem with this method is the mess it makes on components you don't want treated; basically anything you might have to work on in the future. Taking your time with an aerosol gives you the opportunity to avoid these messes along with reducing the risk of damaging any rubber components that don't take well to being exposed to an oil-based product.

Pressure wash the undercarriage thoroughly to remove any stuck-on grit and let it dry overnight. Using jack-stands on a solid, even surface is much safer than drive-on ramps. Having a few pieces of cardboard handy to protect areas you don't want to cover is a good idea. Double coat rust-prone areas, such as those just behind the wheels. And since just about everyone has a supply of disposable masks kicking around, use one.

The best time of year to take care of this job is mid-spring or mid-autumn, when the cooler temperatures will reduce the risk of driveway drips. One of the best brands is Rust-Oleum's Prograde Undercoating spray. It dries fast, usually within an hour, and remains flexible to avoid cracking. Keep it away from common maintenance points like brakes, suspension components, and fuel filters. As one can covers about 15 square feet, take advantage of the fact that it's often available in six-packs. As a bonus, Rust-Oleum's cans are designed to keep spraying when held upside down.

Brian Turner is still kicking and doing his best to put over 4 decades of frontline parts and service experience to work helping you understand not only how your vehicle works, but how to get the best deals when getting it repaired or maintained. He started with us at the Ottawa Citizen and hasn't looked back. He survived rooming with David Booth during his university days and we all admire him for that.

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