Undercoating

Technical Information and Restoration Parts for the 928

Preparation for Undercoating

The under carriage of the 928 is coated with a rubberized preparation that both retards rust and deadens road noise. Often the when repairs are done to the body portions of the car will be under coated with the typical black tar that is used in the U.S. The black tar based system is inferior to the original and will actually trap water promoting rust.

Pressure Washing

The first step in preparing the car for under coating was to wash the under carriage. There is nothing that does a better job than pressure washers and today units are available for reasonable prices at most building supply stores. It is a good idea before washing that the grease and accumulated dirt be soaked in a degreaser. Most auto parts stores carry degreasers.

I tried Simple Green with less than acceptable results. There is a place in the UK that makes bio degradable cleaners. I have not tried these but plan to in the future.

Under Coat Removal

What I was not able to remove in dirt and old under coating with the pressure washer I found almost impossible to get off. I did not want to remove the old body shutz from the factory, but did want to remove the rest of the tar based under coat. I purchased an expensive undercoating removal system from Eastwood that turned out to be a can of oven cleaner and a can of brake clean. This stuff did not work - don't waste your money on it.

I tried Gasoline - never a good approach. This will work well on over spray of new under coat but does not touch dried under coating. The best results I got was with a die grinder and wire wheel. The die grinder will break up the loose stuff and create a solid base for the new coating.

The idea here is not to strip the car down to bare metal, but to create a clean base for the new coating. A die grinder is a quick way to get there.

New Coating

I was able to find a compatible product at the local body shop supply store. The product is called U-Pol Gravi Tex. It comes in a plastic bottle in white , gray and black. The under coating on the 928 is gray from the factory and over painted under the wheel wheels with the color of the car. The U-Pol gray is a little lighter than the factory color, but creates the same rubberized coating and looks as close to the factory coating that is probably available in an after market product.

The mixture from the container is ready for application with a pressure gun. These are available from places like Harbor Freight and other tool houses. Unfortunately my pressure gun never recovered from spraying contact cement many years ago and would not spray the U-Pol as mixed. The container suggests thinning the product and applying with a siphon gun. 50% dilution works great and gives great control. An advantage here is a thin coat is just what is needed to freshen up the original finish. In areas where I needed to patch or fill in the finish I just let the gun flow on a thick coat and then blew a lot of air into it to keep it from running and to duplicate the wavy finish.

Before spraying be sure to mask off everything. As alternative the material could as easily be applied with a sponge directly from the bottle.

The finished product looks nice and creates a sealed surface that is easy to keep clean with regular washing. This coating can be painted but since the interior is gray the contrast is a nice look. There is still a lot of detailing to do, but I am saving some of this for the transmission pull. For now I am ready to put back on the gas tank and get the car running again.