My friend beadblasted the battery tray in his car, then cleaned it with metal prep and painted it with POR black. He just noticed that after a year the POR is flaking off and powdery rust is underneath. The car is rarely driven in the rain, and he was pretty sure he cleaned it effectively. {the only thing I probably would have done differently is that I would have washed it with baking soda before I even beadblasted it.}

Granted, a battery tray is one of the harshest environments on a car but so much for POR being indestructable.
I thought it was ironic that POR uses a battery tray on their print ads.