Ecoat Clinic: Topcoat to Increase UV Protection

Q. We are using a cathodic epoxy electrocoat to prime our steel parts. Now our customer is asking us to start using an additional topcoat to increase UV protection from the sun. What paints are the best topcoats for cathodic epoxy electrocoats? Are there any paints that are not compatible or should not be used over the epoxy electrocoat?

A. Epoxy resins are the most widely universal primers available today. They are compatible with just about any topcoat technology available. 

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There are tens of thousands of successful industrial applications today using topcoat epoxy electrocoat primed parts with powder technologies such as polyester, hybrid and acrylic as well as with liquid paint technologies such as urethanes, acrylics, polyesters, alkyds and others. The liquid paints include both air-dry and thermoset technologies.

Thermoset topcoat paints (in both liquid and powder forms) typically provide stronger inner-coat adhesion to epoxy electrocoated primed parts than air-dry or force-dry topcoats. Typically, they are the preferred topcoat of choice when extreme performance is needed. The adhesion strength to the primer is significantly improved when the topcoat resin uses heat to polymerize and crosslink as compared with air-dry materials, where they only need dry time to cure. 

When air-dry alkyd topcoat paints are chosen, significant testing must be carried out to ensure that good adhesion exists and that the alcohols used in the alkyd paint formula are chemically compatible with the epoxy resin in the electrocoat. This is simply done by topcoating several electrocoated panels under several cure conditions (under-cure, normal cure and over-cure), topcoated with the alkyd paint and determining the degree of adhesion between the primer and topcoat.