I have used POR-10 and POR-15 many times and have had no problems.
Freshly blasted shiny metal should not be covered with POR-15 (this is described in several places on their website).
POR-15 that might see the light of day should be covered with a sealing paint (an enamel, I prefer an epoxy or a urethane).
The outer roof of my Suburban is the largest surface that I have painted with POR-15 (it was actually POR-10 at that time, over 7 years ago). The roof was heavily rusted and pitted. We covered the POR with an epoxy primer, a sealer surfacer and then a single-stage, two-part urethane exterior enamel (PPG DCC). This has held up completely after 7 years (and, much sunshine, and, I regret to say, some substantial rainstorms). I have never waxed the surface and I clean it thoroughly one a year.
The underbody of my cabs and fenders get coated with POR-15.
I am using it on two restorations - on metal and on bed-boards.
