I'm wondering if you don't let the POR-15 dry all the way and coat it with mud you might end up with solvent poping.Not too sure we have the same test going. I'm going to put 2 coats of black por15 on a rust piece and put bondo on it when its still a little sticky but can still touch it without tranfering to you finger. Then I'm going to let it dry overnight and scuff it with #150 and put bondo on that then give it all a stress test.