I've been using the Eastwood rust encapsulator, which is their answer to POR15 and probably is pretty similar. I have been epoxy-priming over it, after checking with the Eastwood customer service. The adhesion will be that of the encapsulator, not the epoxy primer, which is to be expected, but I don't foresee any other problems. The alternative, after all, is to take off all the oxide, which either won't happen or will require sanding off a fair amount of good metal. So, I think it's OK to do, in view of the realities of repainting an older car.