Painting Over Zinc Chromate


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Q. I run a custom painting operation. We have been asked to paint some aluminum aircraft structural parts. What is the best method of painting over 6061 Al that is covered with zinc chromate? Should I use a wash primer or some other type of primer? Also, some of the parts have areas that the customer wants to look like polished metal. Is there an easy way of removing the zinc chromate coating from certain areas of the aluminum in order to have a polished metal finish?  R.F.


A. I hate to do this, but I’ll answer your question with a question. Why aren’t you using your customer’s painting specification? The military has rigid specifications for painting aircraft components. I’m sure commercial manufacturers also have specifications. Did you ask?
The zinc chromate pretreatment is a perfect pre-paint treatment for aluminum. Degrease the zinc chromate to remove oily soils and paint over it. There’s no need for a wash primer. Wash primers, especially the acid activated type, are considered pretreatments and should only be applied directly to bare metal. You can apply any type of primer over the zinc chromate. 
As far as removing the zinc chromate, I don’t know of any procedure that is easy. You can remove the zinc chromate by abrasion in selected areas, being careful not roughen the surface since you intend to polish it.