Tacky Spots In Paint Film

Question:

We are a shop that specializes in metal fabrication for business machines. We spray a variety of paint types and twice in the recent past have experienced a problem that has us baffled. After a six-stage zinc phosphate pretreatment we spray a polyester high-solids paint and cure for 25 minutes at 325°F. The parts exhibit soft tacky spots quite often as large as four inches in diameter. Around these spots the paint is cured to a H-pencil hardness. The tacky spots do not show a color difference nor evidence that a chemical has dripped on them. We are able to scratch down to the base metal with our fingernails on these tacky spots.

1. We have discussed this with our paint vendor and were told the paint will not cure on dirty metal or metal with an alkaline pH.
2. We have investigated the following without pinpointing the problem:
3. The raw metal is oily, but not to excess.
4. Water break cannot be detected throughout the washer.
5. Parts pulled out of the washer before the zinc phosphate stage show stain- ing, but it is not oily and seems to be in the grain of the metal.
6. Before painting, distilled water was put on the dried pretreated parts and the pH was checked with pH paper to be 5.0.
7. After painting, the wet film shows no defects, staining or paint trying to pull away. The problem occurs after baking.
8. We have sprayed test panels and pur- posely put some of the condensation by-products from our oven on the wet paint film and cured these panels. The paint is stained but cured on these panels
9. Re-baking the parts with the tacky spots doesn’t help either.

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Thanks. B. E.

Answer:

High-solids paints are not very forgiving of poor surface preparation and would show some defect in the wet film before baking. Furthermore, if the surface was not properly pretreated, there would be problems with all the paints you apply rather than a problem with one material. The staining you mentioned in your investigation could be rusting of the metal as received. If that were the case, the paint would cure but would have poor adhesion to the rusty areas. I believe your problem is not the pretreatment, but rather the paint itself or some contamination delivered to the surface of the metal in the spray booth during paint application.