Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

April, 2006

Automotive (Car) Refinishing Paint Failure

Q. With in the last 2 years I had my 1984 Mercedes 380 SL re-painted by a very well known shop in Vancouver Canada. I bought this car from Texas and had it trucked up to Vancouver BC. My problem is that under the paint is starting to look like small pin size bubbles in small groups popping up in different spots on the car. The auto paint shop said that they did strip all the paint off right to the metal then primed and painted. The paint shop scratched one of the groups of pin holes to make it bigger and looked at it through a scope and it looks like small rust (pin) spots gathered in a groups. What I would like to know, Is have you ever seen anything described like this? and if so can you tell me what should be done to fix the problem. They will be re-painting the car in the next few days but I am worried the problem will come back unless it is dealt with the right way, They seem to be a bit unsure.

A. You own a 1984 car that you repainted in 2004, or so. Why did you repaint the car? What was the condition of the paint like at that time? Had the paint worn badly, or did you notice any corrosion coming through the paint before you had it repainted?

There are several possible reasons for the problem you are encountering. The first is surface preparation. Did the body shop in Vancouver do a thorough job of prepping the surface? Do you know if they ever sanded through the old paint to expose the metal substrate? If they did expose the substrate, did they prepare the exposed metal? What primers and topcoats did they apply? How many coats were applied? What was the dry film thickness of the total paint system?

Vancouver is located along the pacific coat, although the city is somewhat protected by islands nearby. I have to assume, although I don't know this for a fact, that marine salts can migrate through the fog and settle on surfaces, such as cars, houses, windows, etc. Moreover, based on the times I have visited your beautiful city I assume that Vancouver has lots of rain and possibly also periods of high humidity. These factors raise the possibility of corrosion on metal surfaces. If the body shop that repainted your vehicle did not provide adequate corrosion protection in the form of primers, base coat and top coat, corrosion can easily set in. If you are starting with exposed metal, you cannot compare the quality of a paint job that is performed by an automotive assembly plant with that provided by a body shop.

Unfortunately I don't have enough information about your case, and so I will simply tell you that in redoing the job the body shop must thoroughly prepare the surfaces, remove all rust spots, prep the exposed metal, apply one or perhaps two corrosion resistant primers, and then finish off with the base coat and clearcoat. Dry film thickness of the total paint system is critical. Please bear in mind that all paints are porous to a greater or lesser extent. Therefore, they have the potential to allow atmospheric moisture and oxygen migrate to the metal surface, corrosion can take place. In repainting your car the body shop must make every effort to retard the ability of moisture and oxygen to penetrate the system.

Unfortunately, no knowing the exact situation I cannot make further or more detailed recommendations.

Best wishes,

Ron Joseph

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