Fish Eyes

Q. I recently have been having a problem with fish eyes when spraying epoxy paint, Mil PRF-22750F, FED color #24052. The paint also will have color float (dark green) after thoroughly mixing (shaken 20 min, then mixed with a dispersion blade) as recommended by the manufacturer. The panels being sprayed are cleaned in a brightener solution and then chrome-coated. Parts are thoroughly wiped down with a solvent degreaser. We have sprayed epoxy enamels before and have not had any issues. When spraying this particular epoxy, we spray two tack coats and one wet coat. The fish eyes appear when spraying the wet coat. I have spoken to the manufacturer, who says we should not have to reduce the material. The viscosity of the admix material is at 58 sec using a #2 Zahn cup. I told them we must reduce the viscosity to at least 27 sec in order to achieve proper flow. (We are using Mil-81772 TII epoxy thinner.) We have purged all our air lines (MEK), change our filters regularly, and use no silicones or oils in our spray booth. We even checked the painters for hand lotions. This is the only paint that we are having problems with. F.M.

A. The condition called fish eyes is described by the Coatings Encyclopedic Dictionary as a “paint defect which manifests itself by the crawling of wet paint into a recognized pattern resembling small dimples or fish eyes.” The defect resembles the eye of a fish, having a mound of material in the center of the depressed area. The usual cause of fish eyes is oil, grease or silicone surface contamination.

The contamination could be coming in your spray booth make-up air. The source could be a machining, sanding, or other droplet- or particle-producing area somewhere in your plant. However, getting fish eyes with only one coating material leads me to believe the contaminant is in that material itself. It could also be present in the reducing solvent. Try painting a panel using the suspect material without adding the suspect solvent.