[MP&C]

In the grand scheme of things, the primer needs to be SLIGHTLY acidic to get good adhesion. Too much acid, like a coating of your phosphoric acid residue under the epoxy, and the acid starts to do what acids do. Typically this would be the deposits left deep in pits (see my post above) since they are difficult at best to remove. This normally results in acid activating after a few months in sunlight and you'll start to see small circular raised areas (about 1/8 diameter and less) in the paint finish as the paint delaminates above this residue from the gases given off by the chemical process occurring..


Too much base, as in you used soda blasting without proper neutralizing, and any SLIGHTLY acidic properties the epoxy USED TO HAVE is neutralized by this presence of a base on the metal surface, where it no longer has adhesion. Hence, your paint will pull off in sheets.


So when you hear me preach media blasting (by a qualified person) over any quick fix rust encapsulator/converter/snake oil to address pits, this is the reason why. Media blasting using garnet or coal slag types (Black Beauty) does not cause a detriment to ADHESION like some of these other processes do. When you have to repaint a car, and all the labor and materials that go with that, you'll get it. Let's just say I get it, and it was a lesson based on the owner's generous application of Ospho that was deposited in pits. I'd say take the learning curve while it's cheap.


Disclaimer: Yes, media blasting will cause stretch damage. If you don't know what you're doing, research, research, research. That applies to whether you are doing it or you're finding someone else to do so. If a DA and 80 grit will remove the surface rust, so be it. If it is pitted, the DA won't address all the damage and you're now onto the next option.