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topic 60552

Clearcoated bronze plate gets black spots

November 6, 2017

Q. Black spotting on .25" thick bronze plate, it shows up on polished surfaces. it almost appears to be staining. we oxidize the machined surfaces, stroke sand raised areas. Multiple washes and pre-treating. The staining appears long after clear coat is applied. it leaves facility clean and clear. It is very random, cannot come up with a solution or 2 commonalities. Very frustrated, I do know the staining is between the clear-coat and polished surface.

chuck clemmons
- Danville, Illinois

December 26, 2017

A. Hello Chuck,
What do you mean by "oxidize the machined surfaces"? Can you provide more details? A couple of ideas come to mind when I read your post.
1.) This condition could be caused by a porous or pitted substrate. Fluids can become trapped in pits or pores and then rear their ugly heads later when the part heats up, for example. The trick is to eliminate all pores and pits, use proper dragout techniques along with rinsing and soaking. Most important is to dry or bake out the parts thoroughly and convectionally. If there is any fluid or even gas trapped, it can be forced out in a drying oven. Perhaps dry or bake immediately before applying final top coat.
2.) Although unlikely, an improper top coat could cause stains. For example, if the top coat is too thick or lumpy or not cured properly, there can be problems but usually the result is a cloudy appearance.

blake kneedler
Blake Kneedler
Feather Hollow Eng.
Stockton, California

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