Fisheyes

Ask an Expert From: Products Finishing, ,

Posted on: 1/1/2004

Question: I am having a problem with fisheyes in a coating application.

Question:

I am having a problem with fisheyes in a coating application. We take 5052 H-32 aluminum, punch it, send it out to the plater for chromate treatment and then mask with tape. When we coat the parts, after their return, we end up getting fisheyes.

I’m trying to understand what fisheyes are. It seems to be some sort of contaminate like silicone that will not wash off in our prewash system. Then as the coating is applied it pulls away from the area much like water on a waxed car. T. L.

Answer:

Without going into the physics of contact angles and surface tension, I hope the following definitions will give you a better understanding of what fisheyes are. According to the Coatings Encyclopedic Dictionary, published by the Federation of Societies for Coating Technology, fisheyes are a “(1) paint defect that manifests itself by the crawling of wet paint into a recognized pattern resembling small ‘dimples’ or ‘fisheyes.’” According to Industrial Painting: Principles And Practices, “A fisheye defect in a paint film is a small depression (crater) with a mound (dome) in the center. The resemblance to a fisheye is the roundness of the depression (the outer circle of the eye) and the center mound (pupil).

Fisheyes in coating films are not quite like water on a waxed car phenomenon. Water on a waxed car forms beads and not a continuous film. In organic coatings, the film is continuous except in the areas of the defect. Fisheyes in coatings are usually caused by silicone or oil contamination on the substrate.

Since your parts were chromate treated before painting, they were probably contaminated after pretreatment and before painting. You must determine the source of the contamination. Did it come from an operation in your plant or was the metal contaminated when it came back from the plater? If it came from your plant, did you do something different during the problem period? Contaminants could be falling onto the parts during the coating process. The compressed air could be contaminated by pump oil or silicone seals. Silicone contamination could be introduced into plant air from personal care products like hand cream and anti-perspirants. They don’t even have to be present in the immediate vicinity of the paint application area. It is well known that airborne contaminants can travel long distances in plant air.



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