[D13]

POR-15 is a water cured urethane (not really a paint). It needs something rough to grab on. It will not stick unless the surface is contaminate free (clean) and rough. That's why it works good on rust (lots of little cracks and pores) and simply does not stick on clean smooth surfaces. It doesn't really chemically bond well, the attachment is more mechanical. It works by sealing the air away from the rust site, stopping firther oxidation.
Muriatic acid on the surface under it created a new layer of damage between what the POR was bonded to and the good metal. Now the POR is no longer 'attached' to the surface and just flakes off. BTDT.