Why Use Epoxy Primer?

Q. I am a development engineer in my plant. I was recently assigned to take charge of the finishing operations. Included in my duties is the responsibility for specifying which paints we use on our products. As soon as I took over I was asked a question I couldn’t answer.

We have been applying a two-package epoxy primer and then a polyurethane topcoat to our products. I was asked by management why we were using an epoxy primer and not a polyurethane primer. The answer: “We have always done that,” was not good enough. They want a technical reason. Apparently the purchasing department has gotten into the act also. They want to know why we are not using a polyurethane primer. Apparently the primer and topcoat come from different suppliers. Buying both from one supplier would give us a price advantage. P.C.

A. The short answer is you use a two-component epoxy primer in a finish system rather than a polyurethane primer because of its superior adhesion. Furthermore, lab and field testing has shown that two-component epoxy primers provide a greater degree of corrosion protection to metals owing to their superior adhesion.

However, I recently attended a seminar where a chemist showed laboratory data indicating certain polyurethane primers exhibited superior adhesion. But this is experimental data, not information on a commercial product data sheet. Again, tell your management folks that you are using the two-component epoxy because it has superior adhesion and provides superior corrosion resistance for your products.