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Pinholes in Paint Film

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Q. I am a manufacturing engineer for a company that makes aftermarket products for the automobile industry. All the parts are produced, pretreated and painted in our factory. We have been using the same solvent-based paints for some time. Painting is done by using conventional air-atomizing spray guns. Recently we started to get pinholes in the paint films. What is the cause of pinholes in our solvent-based paints, and what steps can we take to eliminate this problem?  S.D.

 
 
A. There are several causes of pinholes in paint film surfaces. They can be caused by moisture in spray-gun atomizing air during application, insufficient atomizing air pressure and unclean surfaces. In baked-on castings, they can be caused by gasses escaping from the porous substrate during the thermal cure. Pinholes can also be caused by solvents trapped in the film. A solvent flash-off time that is too short and the use of incompatible solvents will also cause pinholes as will too great a film thickness.
To correct these problems, try the following:
Trap and drain moisture from the air lines periodically.
Adjust air pressure to properly atomize paints.
Make sure the substrates are clean and free from oily soils.
De-gas porous casting before painting.
Allow sufficient solvent flash-off time.
Use only recommended solvents. 

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