Painting Over Zinc

Question:

I currently manufacture a part out of cold rolled steel. The part then goes to my plating source. They rack plate the part (zinc plate). The part then moves to my coating source and they paint over the zinc plating. The problem is that when the parts get back to me they are covered in a rough bumpy coating. This is not acceptable to my end item customers. My plater and coater don’t seem to want to help. They just want to point fingers at each other. Please help. J. S.

Answer:

Some coating resins, certain polyesters for example, are incompatible with zinc surfaces. When applied over non-passivated zinc surfaces they react with the zinc resulting in blisters or pinholes in the cured film. This can occur on galvanized, galvaneel and galvalume surfaces as well as zinc die-castings. There are at least two solutions for this problem, passivating the zinc surface and changing the organic coating. One solution is to eliminate the incompatibility with zinc surfaces by passivation. Application of pretreatments such as chromates, zinc phosphates and certain iron phosphates will passivate the surfaces allowing them to be coated without causing blisters or pinholes. Another solution is to change the coating material to one that is compatible with zinc surfaces.