Flaking Paint

Q. We currently are using a two-part epoxy paint to coat the end plates of the industrial filters we manufacture. We have problems with the paint adhering to these end plates when they are subjected to the pressure and heat required to bond them to our products. Paint appears to be flaking off in some small areas, but we don’t have the problem with all end plates, and we run into and out of the problem continually.

These parts are degreased prior to pretreating in a five-stage iron phosphate system. We maintain a good water-break-free surface on these parts as they come out of the rinse tank, then they travel on an overhead conveyor through a dry-off oven prior to application of the epoxy paint. The conveyor travels through a flash-off zone and then through our low-temperature bake oven to speed up the cure and therefore production.

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Should the two-part epoxy paint be able to withstand a temperature of 450ºF for 3 1/2 min, which is required by the adhesive? What could be causing the spotty adhesion failures demonstrated by the paint flaking off? H.Z.

A. At first glance, you seem to have a good finish system, however it must be monitored to maintain its efficiency. The two-part epoxy paint and most other epoxy paints should be able to withstand a temperature of 450ºF for 3 1/2 min. As I have said before, poor surface preparation is the greatest cause of paint adhesion failures. Inconsistent curing is another cause of adhesion problems with any kind of paint and coating. If your end plates are castings, another cause could be outgassing from the porous substrate when heated by the bonding process.

To find the cause of the problem, check your curing oven temperature and monitor the temperatures and concentrations of your pretreatment solutions.