I believe rather than being a chemical rust treatment, POR 15 works by sealing the rust from the air, preventing further degradation. Therefore it should be fine over the primer as long as it adheres. Its coverage and propensity to avoid tiny air pockets is better than normal paint. The issue, if there is one, will be any air pockets or moisture trapped between the primer that you have already applied and the surface.
When you buy a POR 15 'kit" you get rust converter and surface cleaner included. The trick with POR 15 is to get total coverage and a good 'seal". In my experience once cured it's often tougher than the metal it's painted on!
When you buy a POR 15 'kit" you get rust converter and surface cleaner included. The trick with POR 15 is to get total coverage and a good 'seal". In my experience once cured it's often tougher than the metal it's painted on!