Jun 10, 2016 0 comment

“We manufacture infrastructure water valves. Our valves are ductile cast iron and are shot blasted clean after casting. All the carbon scale is removed from the castings. We are currently coating the castings with Epoxy powder, but we run into problems every once in a while with adhesion. The powder coat peels off with little or no adhesion to the castings. Our current powder coat source always blames our cleaning process, but we have inspected our last few shipments and they are blasted very well. The current powder coat source uses a low alkaline wash with a zinc-phosphate pretreatment. There is a silver, almost graphite appearance to the underside of the paint. Any ideas what it may be?”


“We really do not have enough information to make a definitive conclusion. However, if the surface is as clean as you say and has good surface roughness for the coating to bite onto, then the adhesion problem is either caused by too much zinc phosphate or improper powder cure.”

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