Auto Underbody Sealing

by Jeremy Goodspeed

Weather preserving a new vehicle or restoring a classic, underbody sealing has many advantages. First, it provides a first layer of defense against corrosion and abrasion. Second, It works as a sound deadening material by absorbing road noise and damping harmonic vibrations within body panels. This article will take you through the basics of proper underbody sealing.

Before we start, you must first understand a few things about undercoating. If you apply any undercoating product over rust, the coating will soon fail and the rust will continue, being hidden from view until the metal is gone. All it takes is one tiny pinhole or a crack in the coating to allow moisture through, and all of your best efforts will have gone to waste. Such undercoating can easily develop a crack in a short time through vibration or temperature cycling. In some cases moisture can even penetrate through the undercoating by osmosis. There are a few ways to stop rust, but hiding it under a layer of undercoating is not one of them. If you are dealing with underbody rust, you must first remove, treat and seal the damaged area before undercoating will be effective.

However, let's say that you are preserving a new vehicle or a restored classic and rust is not a concern. The first step is to properly clean any surface dirt from the vehicles undercarriage. Placing the vehicle on jack stands and removing the wheels and tires is the best way to access the underbody. Although a pressure washer with a cleaning solution is most effective, a garden hose with a spray nozzle along with a cleaner and some asorted brushes will clean a vehicle effectively. Areas such as the fender wells are usually the most soiled, however clean thoroughly looking for pockets of dirt. Once the vehicle is clean, re-install the tires and wheels and bring the vehicle indoors.

The next step is to place the vehicle back onto jack stands or a service lift, and remove the wheels and tires once again. While the vehicle is drying, remove items such as spare tires, plastic fender-well linings and pieces that would be easy to remove and best not undercoated.

After the vehicles underbody is dry, we will start masking the vehicle to eliminate the sealer from being applied to unwanted areas. Usually, we will start with the fender edges and rocker panels. 2" masking tape along these edges works well to create a brake line. Next, areas such as shock absorbers, drive lines, and suspension components that should not be undercoated should be masked. This is best to be masked with aluminum foil. The foil can be manipulated into various shapes and can be wrapped around stubborn surfaces without falling off. This approach also saves time and provides a strong barrier. After the underside has been masked, use 12" masking paper along the vehicle edges. This will eliminate the body from receiving any unwanted over-spray.

Before application starts, be sure to use a respirator, replace the charcoal cartridges with a new set and wear it when using any underbody sealer. The vapors are strong and can cause dizziness. Next, use a portable light source under the vehicle to light up your work area. This will show areas where the coating is thin and will need more material applied.

Undercoating is available in two forms; Aerosol or gun applied. Although aerosol undercoating is great for touch-up or doing small areas such a fender-wells, Appling a sealer to an entire underbody is best suited by use of an air powered gun. This type of sealer also has a higher build, better spray pattern and longer protection benefits. Start from one end of the vehicle and work from the center out. By creating a systematic pattern, no area will be forgotten. If spraying only a vehicles fender-wells, 3 liters would be sufficient, however many vehicles can require between 8 to 12 liters to completely cover the underbody. It sounds like a lot of underbody sealer, however keep in mind that this material is a high build product.

After application is complete, allow the vehicle to dry. Plastic based under-coatings dry faster than tar-based products and plastic based products usually are dry to the touch within an hour or so. Once the material is dry, remove the masking film, tape and paper. Allow the vehicle to dry for a couple of hours more before reinstalling plastic fender liners or spare tires. It would be recommended not to drive the vehicle for 24 hours to allow the material to fully harden and cure.