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From: Products Finishing,
David S. Peterson,
What we are seeing is craters in random places in the surface of the painted parts.
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Pretreatment Washer Setup
Q. We are having a problem cleaning galvannealed steel. (See washer set up in the image to the right.)
Plant air is used for removing excess washer solution at the entrance and exit of the dryer, which is set at 350°F. A wet paint is applied and cured at 450°F with a 20 min dwell time.
What we are seeing is craters in random places in the surface of the painted parts. We have tried the following: wipe with solvent (Methyl Ethyl Ketone/MEK); pre-clean with same cleaner (4% by volume and 140°F); increase cleaner stage temperature to 160°F, clean and vacuum dryer and cure oven; and review the gloves operators use to handle parts prior to washer.
No excessive oil, grease or dirt can be found, yet we are seeing 80% rejects on our first spray and then 90% good parts on respray. Do you know of anything we may have missed with the galvannealed substrate metal? R.J.
A. First, let me tell you that is the most complete description that I have received in more than 11 years of writing this column. If I had a first prize to give out, you would have won it.
The cratering defect you are seeing is also called “fisheyes” in the paint world. In this case, the problem is that the paint does not want to wet or flow out in the affected areas. This is due to a local difference in surface tension that is usually caused by insufficient cleaning. How did the additional/alternate cleaning steps affect the results? Did increased cleaning show an improvement? It could be that the lubricant applied to the galvannealed steel is too difficult to remove in a simple 60-sec spray wash.
Zinc-coated steels are generally coated with a corrosion inhibitor that is meant to prevent the development of “white rust” or zinc corrosion product prior to delivery of the material. In some cases, that storage time can be very lengthy, especially for products imported from overseas. If this is the case, the corrosion inhibitor may be similar to a wax-like substance that cannot be removed in the washer described and may need a longer residence time, possibly in an immersion tank at high temperatures.
Alternately, you should also make sure that there is no chance of silicone getting onto the surface. The resultant fisheyes would manifest similarly. Silicone could be coming from several health and beauty products, particularly items like hand creams. These should be completely banned from your shop floor, and workers advised as to the detrimental nature that silicone-containing products have on paint quality. It could be that the second coating is simply “filling in” the areas previously affected.
Last but not least is to investigate the quality of the plant air that you mention is used at the exit of the washer and the dryer. This air should be dried to remove water and filtered to remove oil typical of most compressor systems. This seems like the most likely culprit given the random and spotty nature of this problem.
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