Ask the Expert Question-and-Answer Archive

by Ron Joseph

June, 2002

Coatings for Whiteboards

Q. One of my customers needs to eliminate etch primer from the system. He is painting etch primer then epoxy zinc phosphate primer and the epoxy top coat on aluminium casting. Will there be a failure of the system if we eleminate etch primer or can you suggest me some other coating which I can use?

A. The use an etch primer is really dependent on the substrate. If the aluminum casting is pretreated with an inorganic

aqueous system, such as Henkel's "Alodine", or any other propriety conversion coating, then you will not need, or even want the etch primer.

Unfortunately, aluminum is a difficult metal to paint with success. Adhesion of organic coatings, such as epoxies, is difficult because of a natural oxide film that forms on the aluminum surface. On the other hand, a proper conversion coating

, (etch primers are NOT conversion coatings) such as "Alodine" or any of the other trade products in its class, removes the oxide from the aluminum and then immediately passivates the surface, usually with a chromate-containing oxide.

If your customer wants to eliminate the etch primer, then can he abrade the aluminum surface by grit blasting, or using any other mechanical method to give the aluminum a profile so that the epoxy can grab onto it and achieve some physical adhesion?

The final answer is for your customer to experiment by applying your epoxy primer over his casting, without the etch primer, and determining if the subsequent adhesion is adequate for his purposes. There are several accelerated laboratory tests that can be performed to evaluate the adhesion between the primer and substrate.