Ask the Expert Question-and-Answer Archive

by Ron Joseph

January , 2008

Fish Eye Problem

Q: We're facing the problem of fish eyes on sheet metal. We are using zinc phosphate. Most of  time it appears only on the top side of panels, and other times it appears on all sides of the panels. We are using LPG burner without a heat exchanger.

A: The first task is to identify the contaminant that is causing the fish eyes. Is is oil, grease, silicone, or what?Unfortunately, contaminants that cause fish eyes, such as silicone, can travel in the air over long distances and might not have anything to do with the zinc phosphate pretreatment.

Once you have identified the contaminant, you will need to locate the source. Sometimes one can find the source quite easily and it might be due to an aerosol that you are using elsewhere in the plant, but I have been involved in cases where it was extremely difficult to determine where the contaminant was coming from. In one case, the contaminant appeared to have come from a neighbor's factory, and entered through the windows or doors of the paint facility.

I know of a large aircraft facility that spent tens of thousands of dollars trying to locate the source of the fish eyes, but the last time I heard of the situation, they had still not solved the problem. 

Please get back to me if I can be of further assistance.

Regards,

Ron Joseph