Car Undercoating Pros and Cons

by Don Elliott on January 17, 2012

Car undercoating is a thick black coating applied to the underside of cars and trucks to protect surfaces from road salt, stones, debris, and other corrosive substances. Car undercoating is a generic term for a wide range of products. These products are usually petroleum based with rubber, silicone, ceramic, asphalt, and fiberglass added in various combinations. If you aren’t sure what type of car undercoating product would be best, your auto repair mechanic can recommend a type of car undercoating.

Undercoating is often confused with rust proofing products. Rust proofing is applied as a thin clear coat, while car undercoating is a thick tar-like product.

Car undercoating can be sprayed or brushed onto your car. For the underside of cars, spraying is definitely the easiest way to apply the product. Auto parts stores sell car undercoating products in large spray cans, costing about $50 to do a typical car.

Professional applications are generally much more effective than the do-it-yourself jobs. First, it is necessary to do a complete undercar power wash to remove any loose dirt and grime. Any trapped particles will provide an area for the car undercoating to flake off and eventually become a place to trap road salt and moisture. Second, an even smooth application will provide the best results. Thick blotchy patches can clog manufacturer installed drain holes, gum up moving parts, and add unnecessary weight to the vehicle.

Car undercoating does not stop existing rust. At the very least, loose rust has to be removed and should be treated with a rust inhibitor before the undercoating is applied. Undercoating is often used to slow down the rusting process on older cars, but it must be reapplied periodically to have any effect.

Most new cars have car undercoating applied at the factory. That does not keep new car dealers from offering it up as an after-sale product for new car buyers. Unfortunately, this is one area where new car dealers will take advantage of naïve new car buyers. They will charge up to $1,500 for a product that is really not necessary.

Car undercoating offers an additional advantage—soundproofing. The coating quiets an otherwise noisy ride, typical of less expensive car models. Used car dealers have been known to undercoat older cars, making them ride more comfortably on the test drive. Undercoating is also a way to give the underside of a used car a clean fresh appearance at very little cost.

Opinions are mixed about the value of car undercoating relative to the overall car value. On a new car, it is probable that car undercoating can do more harm than good. On an older car, undercoating needs to be reapplied periodically to maintain its benefit. A poor application over a dirty undercarriage will eventually provide nooks and crannies perfect for trapping moisture and road salt spots that can eventually rust. Check with your car repair shop to see if they’d recommend a car undercoating for your vehicle.

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