Automotive Mileposts
Image: Rust holes in metal

Rust is common underneath factory vinyl roof coverings. The vinyl and padding traps moisture, keeping it in contact with the metal. Since the car was being built with a vinyl roof installation, many are not painted in this area, an invitation to corrosion. Factory primers were no match for time and moisture.




No matter what you call it, it is the #1 enemy of old cars.

What it is, how it happens, and what can be done to fix it...

What is rust?

Rust is a general term given to the reaction of iron and oxygen when in the presence of moisture. Iron oxides—most often red oxides—are formed when iron, oxygen, and moisture are all present. The process is commonly known as rusting, which describes the corrosion of iron and its alloys. Over time, all untreated iron masses exposed to oxygen and water will convert to rust, eventually disintegrating completely.

Rust is easily identified in its early stages as an area of rough, brown-to-red colored metal. In later stages, the area begins to flake or holes develop in the surface. Once it begins, it must be dealt with or it will continue to progress.

Chemical reaction

When iron is in contact with oxygen and water, or any one of a number of strong oxidants or acids, a corrosive process begins and rust forms. Under certain conditions, an electro-chemical reaction can occur, such as when salt is added to the moisture, which speeds up the corrosive process. The electrochemical process begins when electrons from iron transfer to oxygen. This corrosive process is affected by water and accelerated by electrolytes. This is why automobiles in areas of the country where the roads are regularly treated with salt in the winter to melt ice seem to rust so rapidly, the salt acts as a fourth agent to speed up the corrosive process.


Rust is permeable to both air and water, so as long as there is exposure to these two things, corrosion progresses. However, if a coating is applied to the iron that prevents exposure to either air or water, rust formation can be precluded, as both are required for the process to occur. For instance, stainless steel forms a passivation layer of chromium (III) oxide, which interrupts the process. Aluminum, magnesium, titanium, and zinc all have similar passivation characteristics.

The process of galvanization applies a layer of zinc to the metal needing protection, either by a process known as hot dip galvanizing or by electroplating. Zinc is often chosen due to its reasonable cost and its strong adherence characteristics to steel. Cadmium plating is another alternative, and while it is more costly, it also affords better protection in severely corrosive environments, such as long term exposure to salt water. Galvanization can fail where there are seams, joints, holes, or wherever the metal is scratched. In this instance, some coatings add aluminum to the process since aluminum will creep to cover scratched or damaged areas and provide protection for a longer period of time.

Other ways of rustproofing include cathodic protection, which inhibits corrosion on steel that's buried or immersed by supplying an electrical charge that can completely stop the electrochemical reaction from taking place. You may have heard of kits that can be attached to automobiles that stop rusting by using a sacrificial anode that's normally made of a material that has a more negative electrode potential than the metal being protected. The anode sacrifices itself to save the surrounding metal.


The formation of rust can be controlled with paint-type coatings that provide a seal and prevent air or water from coming in direct contact with the metal. Some paints feature a special formulation which actually reacts with the corroding metal to stabilize it, then when dry the coating seals the metal off from further contact, thus effectively interrupting the corrosive process.


To stop the corrosive process, you must remove one of the three things required for rust to form: 1. iron, 2. oxygen, or 3. water. Obviously, iron is what we're trying to protect, so air and water are the real enemies here. Once rust has started forming, quick action is necessary to bring it to a halt. If you have a project car with rusty areas, treat the rust immediately, even if it will be years before you get around to working on the car. In the long run, it will cost less money to repair and require less body work than if it sits untreated for a period of time.

Rust can and will move to adjoining areas, given enough time. Treating those areas beforehand will save you a lot of time and expense later on. For additional information, consult our Classic Car Rust Repair page.

Products to treat rust

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