So, the paint came off the PPG Epoxy Primer, but the primer stayed on the Rad Support. That tells me that the Epoxy Primer wasn't flashed enough, 2 hours is a guideline. Flash times are dependent on temperature, air flow and volume (the amount of primer you put on). If it was cool, the solvents where still in the Primer, when you painted over top...the paint cured before the solvents where out of the epoxy...when they tried to release themselves, they took the paint with them. Lack of air movement will not allow the solvents to release, to much air movement will cause the primer to cure on top and not all the way through...causing the paint to come off when the solvents finally do release. Same with volume of paint. The more primer you put on, the longer your going to need to wait for it to flash and top coat.
Anyway you look at it, 90% of peeling paint is caused by trapped solvent, 9% is cause by poor mechanical adhesion (sanding), but the primer stayed on the rad support, so mechanical adhesion isn't the issue. The other 1%...it was your turn...sometimes it just happens but even then, it's probably something that happened in the painting process.