By Tom Pawlak — GBI Technical Advisor
In the past, we’ve recommended applying an alkyd base primer over well-cured, clean, and sanded epoxy surfaces. We wanted to know how this primer compared to the newer, fast drying primers now available. We recently did a study of various house paint primers over epoxy to support the growing number of customers using WEST SYSTEM epoxy for building restoration.
The test identified primers that adhere well to cured epoxy and require only one hour drying time before finish paint can be applied. Compared to typical primers, these products appear to be much less sensitive to surface preparation and application technique over an epoxy surface. Some of the quick set primers were just recently approved for use in exterior applications by their manufacturers. We did not test these over bare wood. This test focused only on the adhesion and drying effectiveness of these primers over epoxy.
We tested the various paint primers using the ASTM-3359-83 test. It involves scoring the painted surfaces with a razor blade. Two 2″ long cuts are made, deep enough to pass through the paint. The second cut bisects the first cut at a 45° angle. This point of intersection creates an ideal place where the paint will peel off if adhesion to the underlying surface is poor. Special tape is applied over the scored surface and pressed against the paint to ensure thorough contact. The tape is pulled quickly from the surface with a sharp tug. If paint adhesion is poor, a lot of paint is pulled up with the tape. If adhesion is excellent, no paint is transferred to the tape. We rated the performance between these extremes using a scale of 1 through 5, with 5 representing excellent adhesion. The results are shown on the chart below.
All of the quick-set primers tested had excellent adhesion to epoxy-coated surfaces that were sanded or washed and sanded. In addition, each of these quick set primers could be recoated with finish paint the same day, saving days on the job.
If using alkyd based primers, clean the surface and sand it aggressively, then apply primer in thin coats. A thick film will take days to harden before a top coat can be applied.