What brand of primer are you using? Several years ago we tried to switch away from DP epoxy primer by PPG to the SPI equivalent epoxy primer, which is somewhat cheaper. As soon as the weather changed to cooler temperatures we had the same problem. And it seemed that if the substrate was not warm at the time of painting, it did not help to heat it up later because the SPI epoxy did not want to cure in a reasonable time. We went back to DP epoxy and the problem went away. I could not justify tying up the booth waiting for the primer to dry.
Mark
