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Automotive paint question

Automotive paint question

Automotive paint question

What is a good way to determine porosity of dried paint? I have seen many cars with pimples in paint caused by entrapped water, either from the painting system or not properly dried water based paint, or from car covers that maintain moisture. I do not know what kind of paint or clear coat was used in any of the problem cars.

RE: Automotive paint question

I'm not sure, but maybe you'll find an answer in EN 15384:2007. It is about determining porosity of internal lacquered coatings of aluminium tubes. It may help, but I hanen't any acces to this standard, so I'm not sure.


RE: Automotive paint question

You could look at a sample of your paint using a microscope and from experience you could start to tell which are good and which could potentially give you problems. Some people use a "sealer" coat of primer which is just a thinner coat that is supposed to reduce the porosity to seal everything solid. Many times when you get moisture bubbles it's because the moisture is getting in from the back side of the metal. This is the main reason why cars with bondo repair end up bubbling where the bondo was applied.

RE: Automotive paint question

In the examples I am talking about there was no way the moisture was getting in from behind. I did an experiment with well dried alkyd enamel and had it lift in pimple form from maintaining moisture on it. The paint had about 4 months to cure and had a thin sheet of plastic covering an area that held moisture on the paint. There was no rust under it.
Other example was a car deck lid that blistered in an area that might have had an extra water bath from a canopy that leaked on it. Close inspection showed that the lid was a repaint, no metal was visible when bursting the small pimples.
The pimples formed after a good freeze temps into the 15 deg F range. It could have been from the water bath or from incomplete water base paint drying, I don't know. Just saying it happens.

RE: Automotive paint question

Way back when I was involved in shiny metal things I was told that almost all paint systems are porous, and that they prevent oxidation of the substrate by an electrolytic charge.

And that is /everything/ I know about that.


Greg Locock

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RE: Automotive paint question

Water in wet paint is a completely different problem from water blisters that form after the paint has been completely dried. Water blisters are caused by osmosis where water vapor passes though the paint film and is attracted by salts or some other substance that creates osmotic pressure. This pressure then causes blistering through the accumulation of water behind a non-porous film. Porosity in the film will actually prevent these blisters. Google "osmotic blistering".

RE: Automotive paint question

I have checked out what you suggested. I didn't find much on how waxing helps or hinders. Since wax should close up the pores, I know it helps shed water, does it also help trap moisture?

RE: Automotive paint question

The only true solution is proper paint, surface preparation, application, and curing. However, anything that minimizes exposure to water and humidity will help. Understand that exposure to liquid water is, for many purposes, the same as 100% relative humidity in air.

RE: Automotive paint question

So what is your thoughts on waxing the paint? What does it change? Will a car cover remove the wax from motion of wind and rain? And I agree but what a way to determine that a part has been repainted, and with sub standard paint. A factory paint job is probably the best there is, well on some cars anyway.

RE: Automotive paint question

Who the heck uses an alkyd on cars?

Alkyds are never going to work in a continuous wet environment.

RE: Automotive paint question

I am not aware of alkyd being used for many applications on automobiles, mainly specialized functional coatings on small parts. Definitely not for the primary exterior surfaces (urethane clearcoat) or underbody (epoxy electrocoating).

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