Foreword: Rather than to address “undercoating” as a general topic of discussion, I’ve tried to keep the focus upon undercoating as it related to Volkswagens through 1979 and as it relates, now, to the vintage Volkswagen hobby.
Undercoating has around for years and years in the world of vintage Volkswagens.
The theory behind undercoatings is that a barrier could be created to prevent the infiltration of moisture. Undercoatings themselves had no rust-inhibitive qualities. They simply have been intended as a barrier.
VW dealerships sold the service to new car buyers as a preventative measure to guard against rust. It was a money-making operation and dealers loved it. Especially was it offered in the colder climate States and especially where salt was used on icy roadways.
Recently, I spoke with a former VW trained specialist. He described the undercoating procedure as he observed it. He said that the dealership where he worked had one bay with a lift, “in a dark corner”, where the “nasty” undercoating took place. He told me that it was part of a money-making effort by dealerships in the make-ready department. Undercoating was applied using a hose and gun working from a 30 gallon barrel of material.
I have had my doubts over the years about its effectiveness in sealing the undersides of a vehicle as a moisture barrier. Here’s why. I was in the painting industry for almost 30 years. If there is a coating, I likely have seen it or read about it. In my experience, despite all claims to the contrary, coatings will fail. There is no “eternal” coating. I’ve heard claims that “you’ll never have to paint again”. Why can’t this be true?
It can’t be true because of expansion-and-contraction problems. When the base material—wood, metal, plastic—expands or contracts, the coating is going to suffer, eventually. Some coatings are better suited than others. But the fact of the matter is that coatings fail.
Metals, especially, are given to fluctuations from heat and cold. They will expand and contract more, and more quickly, reacting to weather and usage conditions.
What’s another problem? It’s the fact that the underside of a vehicle is not a continuous sheet of metal. Not at all. The undersides of vehicles are composed of pieces that have been fitted to form a unit. This could be through a continuous weld or spot-welding or with nuts and bolts and washers. There are joints. Every place where there is a weld or a nuts-and-bolts joint, expansion rates will differ.
As well, the application of undercoatings must completely encase all of this in order to form a viable covering—it must be seamless. This doesn’t happen.
The next issue is that undercoatings historically were shot onto factory painted surfaces. In order for a coating to adhere, there must be the possibility of adhesion. Slick surfaces will not offer such adhesion possibilities. As a result, I have been able to remove portions of undercoatings on Volkswagens simply by using compressed air. Sometimes, I have been able to remove it in sheets, simply because of the lack of adhesion. I can imagine that vibration over the years helps to loosen poorly adhered coatings.