Dirt in the Paint Film

Q. We manufacture metal and plastic logos and product branding items in one of our overseas plants, where we have 30 paint booths, half of which use traverse-type auto spraying equipment. We have a couple of particularly difficult problems with products that are coated with glossy black and glossy red paints. We have been getting a very low product yield due to what we are calling contamination in the paint film. This could be dirt or other defects that look like dirt. We are not sure where it comes from.

In reviewing some information on spray painting fundamentals, we know there are several areas such as distance to work, viscosity and pressure used that we do not follow. We are thinking we need to get our paint process back to the fundamentals to help us to make improvements. We are looking for someone to review our process and come up with a plan to make these improvements and help improve our overall product yield.  —M.H.

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A. Your problem doesn’t sound like you are straying from the basics of paint spraying. It is a problem common in industrial paint lines. Hiring a paint consultant charging an hourly rate plus expenses can be very costly, and that money could be better spent elsewhere.

You or one of your people will have to do some investigative work to:
1)  Identify the contaminant, either microscopically or chemically.
2)  Find the source of the contamination.
3)  Change the contaminating process or eliminate it.

The contaminants could be dirt in the paint itself as received from the supplier, or they could be introduced in your plant by poor housekeeping or using dirty containers during paint transfer. They could also be pigment agglomerates (several pigment particles stuck together to form a clump). In any case, this problem can be solved by filtering the paint using in-line filters or filtering before filling clean paint supply pots.

The contaminants could also be airborne particles entering the spray booth in make-up air. They can travel long distances in shop air. Common contaminants are grinding particles, sanding particles, shop dust and outside dust. Filtering the spray booth make-up air usually solves the problem. It is common practice for spray booths painting critical parts to be fully enclosed.

The contaminants may already be on the parts before they are painted. If this is the case, some kind of pretreatment is required, such as an air blow-off or an aqueous cleaning stage. You have to determine how the parts were stored and for how long before painting. Parts are often stored in open bins where they can become contaminated with shop dust or shipped in open bins where they can become contaminated with road dust.

If you can’t do the investigation at your overseas plant, I suggest you have samples of the poorly painted parts shipped to you. You can accomplish an analysis of the offending paint film either in-house or by sending the parts to a testing lab. Either course of action will be less costly than hiring a consultant.