Projects & Repairs --> Maintenance --> Under Coating

Rust-proofing a car with spray cans of undercoating.

Applying Undercoating To A Jeep Cherokee

Prevent Rust On A New Car By Protecting The Sheet Metal On The Underside.

In This Article:

The underside of the car is pressure-washed and allowed to dry. The car is raised up safely and undercoating is sprayed onto the sheet metal on the bottom of the car.

Related Articles:

Skill Level: 2 (Basic)

Time Taken: 3 Hours, Plus Drying Time

By Jim Wilk, Licensed Auto Body Technician,
And Bruce W. Maki, Editor.


If you live in the northern United States or Canada, you know how road salt can make cars rust quickly. Some people like to apply undercoating to the bottom body surfaces of a new car, before the road salt has had a chance to begin the corrosion process.

Those of us that work in the auto body business have seen many cars that essentially rust from the bottom up. Since road salt is often mixed with sand, there are a lot of abrasive particles on the road that get splashed onto the bottom of cars, causing nicks in the factory coating. Any nicks or scratches in the metal coating can rust, and salt accelerates the corrosion process. If steel is kept completely covered with a non-porous coating, it won't rust... but it's difficult to keep that coating perfect.

While undercoating a used car is not usually recommended, a customer who had purchased this 2003 Jeep Cherokee in the desert area of Arizona asked us to apply undercoating to the vehicle to keep the bottom sheet metal from rusting in our Northern Michigan winters.

Before applying undercoating, the surfaces must be clean and dry.

The day before, we lifted the car on our shop hoist and power-washed the underside. Then we let it dry overnight.

Then we hoisted the car again to apply the undercoating.

Lifting car on hoist to apply undercoating to bottom.

Of course, the average do-it-yourselfer probably doesn't have a hoist in their garage. But, some cities have businesses that rent hoists by the hour.

It's possible to undercoat a vehicle that is raised up on jack stands. You don't have as much room to maneuver as the hoist, but it's been done.


Areas on the bottom of a car to apply undercoating to.

The red arrows point to some of the not-so-obvious spots on the bottom of the car that need to be undercoated.


Some of these areas are hard to reach because they are behind the exhaust system or drivetrain parts. Areas to be undercoated.


Sheet metal on the bottom of car to be undercoated. Other areas are more obvious, such as the C-shaped frame channels.


These brake lines and fuel lines would be rusted after just one winter up north, but this 7-year-old car had virtually no rust on the metal lines. Metal brake and fuel lines on bottom of car to be undercoated.


Suspension area of car. It can be tricky to spray undercoating behind the suspension control arms.

The control arms themselves may not need undercoating if they are made from heavy steel forgings, but these control arms are made from steel stampings, so we coated them.


We used 10 spray cans of 3M brand rubberized undercoating on this car.

We ended up applying 3 coats before all the cans were used up.

We started spraying at the front of the car...

Spraying undercoating on the bottom of a car.


Undercoating a car with spray cans of rubberized undercoating. ...and worked towards the back.

We didn't remove the wheels because the rear wheel wells had undercoating applied at the factory, and the front wheel wells have plastic shrouds above the tires.

On some cars it might be helpful to remove the wheels, especially if the car was only placed on jack stands. Then you would have more elbow room to work.



Follow the instructions on the spray can. The instructions should describe how much the can needs to be shaken, how far to keep the spray nozzle from the car, and how long the material takes to dry.


After applying the first coat of undercoating there were still some spots that didn't receive a thorough coat.

After the first coat had dried for about 15-20 minutes, we applied a second thin coat.

Bottom of car after applying undercoating.



Undercoating materials are often made from petroleum products, so they can be flammable and/or harmful to your health. Use only in a well-ventilated area.


Undercoating a new car. We did this job with two people... one held a flashlight and looked for thin spots and bare spots while the other used the spray can.

It can be hard to find some of the thin spots because you are looking for a not-so-black area amongst all the black areas.


We avoided spraying undercoating on the heat shields (red arrow) above the exhaust system. Avoid spraying undercoating on heat shields.


Thin areas in undercoating need more coats. After the second coat there were some thin areas that showed up as the liquid dried.


We sprayed the underside of the body around the hump above the rear axle. We also sprayed the suspension control arms and any metal brake lines.

(The red arrow indicated the back tire, the yellow arrow points to the rear axle.)

We also applied undercoating to the areas where the shock absorbers mount to the car body. If these areas rust, it can be expensive to repair.

Rear suspension of car.


Jeep Cherokee. This 7-year-old car looks quite new, so it's worth spending a few bucks to prevent rust on the underside of the body.


More Info:

Tools Used:

  • Power Washer
  • Shop Hoist
  • Flashlight

Materials Used:

  • Rubberized Undercoating, 10 Spray Cans

Related Articles:



Home  |  What's New  |  Projects & Repairs  |  Links  |  Contact Us
Search Page

Copyright 2010
All Rights Reserved

Written April 15, 2010