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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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