Paint Blush

Question:

I work for a door manufacturer as a process engineer for steel doors. When we paint our doors, we sometimes have a phenomenon that is called “paint blush.” Our doors are made using phosphated EG (electrogalvanized) steel with a chromate rinse sealer. The flash oxidation has had us stumped for quite some time. We paint some doors grey. When this issue pops up the doors are covered with white spots. I have been working to try to figure out the cause. The question has been is it the steel, the paint, or an interaction of both? Here’s some more general info: The paint is water based, zinc is at 0.051 oz/sq ft, phosphate is 75–125 mg/sq ft and chromate is approx. four mg/sq ft.

I run salt spray tests on the unpainted coated substrate for days and there are rarely signs of white oxidation, yet we paint the steel and this phenomenon occurs in the half hour paint/drying cycle.

Do you have any ideas as to what is really going on? Is this indeed zinc oxide that we are seeing? Are there chemicals that might promote the formation of zinc oxide? M. K.

Answer:

It sounds to me like your paint is reacting with zinc on the electrogalvanized surface, forming zinc oxide. But you already know that. Yes there are chemicals that promote the formation of zinc oxide and they are probably present in your paint. The Painting Clinic is not the forum for a discussion of chemical reaction mechanisms. Since you are applying your paint direct to metal (without a primer), you don’t want the added expense of using a primer as a barrier coat (that would solve the problem). Another solution is to replace your paint with one that is compatible with electrogalvanized steel.