DIY Undercoating

by William Zane

Undercoating is a thick, tarlike substance that is sprayed on the bottom of a car. While undercoating helps prevent rust, its primary purpose is to deaden the noise from rocks and other road debris. If undercoating is applied over rust, or if moisture is trapped between the undercoating and the chassis, it can make the rust worse. Rustproofing should be applied before the undercoating if desired.

Types of Undercoating

On the low end are brands like Rustoleum, which is available from chains such as Wal-Mart for about $5 per spray can. On the high end are brands such as Wurth, which offers undercoating in a spray can for about $13. Other types of undercoating come in larger containers and are applied with a spray gun. For home undercoating, the spray can is the most convenient method.


Jack up the car at all four corners and place it on jack stands so you have access to the underside and the wheel arches. There may be undercoating already in place. You can either chip off the old undercoating, which can be difficult and time-consuming, or use a wire brush to remove dirt and then re-coat areas that need it. If you are undercoating an area that has not been treated, use wire brushes and wire wheels on a power drill to prepare the surface. Once the area is free of dirt and rust, wipe it down with a paint preparation solvent to remove grease and oil. Thoroughly dry the area. Once everything is clean, mask areas like fenders where you do not want undercoating. Mask the suspension, bolts and wiring as well. If undercoating gets on these areas, it can make maintenance difficult.


Spray the undercoating from side to side 10 inches from the surface. Apply at least two layers, allowing time for the undercoating to dry between coats. Most undercoatings dry into a flat black shade. You can also spray the undercoating with a color to give it a more finished look. The chemicals in undercoating are very strong and can cause respiratory and nerve problems if fumes are inhaled. To prevent exposure, apply the undercoating outside or in a well-ventilated area. It is also recommended that you wear a respirator and rubber gloves.


About the Author

William Zane has been a freelance writer and photographer for over six years and specializes primarily in automotive-related subject matter among many other topics. He has attended the Academy of Art College in San Francisco, where he studied automotive design, and the University of New Mexico, where he studied journalism.