I´m restoring the undercarriage of my 1974 Cadillac Coupe DeVille. See below. As I want to do it as perfectly as possible, I wanted to remove all the undercoating which was applied when the car was new. I do not like rubberized undercoating, as you cannot see whats going on underneath. It can trap moisture and once it flakes off due to corrosion, there is already some major damage in the metal underneath.
There are different methods for removing the undercoating and most of the methods are a pain in the a**. This stuff can be removed with a scraper and chemicals, or with heat and a scraper. Both methods will damage the metal underneath and scratch the paint on the underbody, and it takes forever…
As I did not want to use any of the methods mentioned above, I decided that I wanted to try dry ice blasting.
Dry ice-blasting is a form of carbon dioxide cleaning, where dry ice, the solid form of carbon dioxide, is accelerated in a pressurized air stream and directed at a surface in order to clean it. An alternative media for non-abrasive blasting is water-ice, known as ice blasting. The method is similar to other forms of abrasive blasting such as sand blasting, plastic bead blasting, or soda blasting but substitutes dry ice as the blasting medium. Dry-ice blasting leaves no chemical residue as dry ice sublimates at room temperature. Dry-ice blasting involves propelling pellets at extremely high speeds. The actual dry-ice pellets are quite soft, and much less dense than other media used in blast-cleaning (i.e. sand or plastic pellets). Upon impact, the pellet sublimates almost immediately, transferring minimal kinetic energy to the surface on impact and producing minimal abrasion. The sublimation process absorbs a large volume of heat from the surface, producing shear stresses due to thermal shock. This is assumed to improve cleaning as the top layer of dirt or contaminant is expected to transfer more heat than the underlying substrate and flake off more easily. The efficiency and effectiveness of this process depends on the thermal conductivity of the substrate and contaminant. The rapid change in state from solid to gas also causes microscopic shock waves, which are also thought to assist in removing the contaminant.
Unlike abrasive media blasting you can not remove rust with dry ice blasting.
I found a company through the website of a classic car club, which is only a little over an hour away. I wanted to have it done before the first snow and as soon as possible, so that I could continue working on my car during the winter. The owner of the company did a great job. He is a very friendly and competent gentleman and it was a pleasure working with him. Here is a short video how it was done: (make sure to watch in HD for proper image quality)
The car was lifted with a forklift and then covered in plastic.
One of the rear wheel-housings before blasting - you can see the undercoating everywhere. The same wheel housing after blasting - all the original paint was still very well preserved under the undercoating. As you can see originally the wheel housing was brown. During painting a lot of overspray from the body landed in the wheel housing. Then Cadillac added rubberized undercoating to protect the metal from stone chips and to keep the car more quiet. Luckily there is no rust to be found anywhere. During blasting. The original brown paint showed up underneath the undercoating. The paint is still in perfect condition.
The original paint showed up underneath the undercoating in the wheel housings as well. After cleaning - it came out really nice! I will touch up some areas and conserve the others with a special transparent coating. I´m very happy with the result of the cleaning and will now have to protect everything and paint some areas.