Long story short, I'll stick with epoxy primer as it has every good point of POR-15 (except painting over rust, which just plain shouldn't be done) and fewer of the downsides. It is not expensive, requires no binder step, but is still not UV resistant and shouldn't be on external parts without a topcoat.
End Rant.
Mitch . . . I am not seeing any rant here whatsoever. Metal is inherently ductile in nature so there is no coating - Read NO COATING that will permanently prevent rust in ferrous metal subject to ebb and flow dynamics; which is in fact all metals subject to changes in temperature. Time is not on our side when it comes to steel and iron - and the time frame is shortened the more the metal is flexing.
I agree with your assessment on epoxy base coats - it is indeed the way to go with new metal or rust free metal. But in the restoration world totally rust free surfaces can be hard to come by. This is the niche of POR15. It reduces effort (labor) in preparation - but it doesn't eliminate it. Slathering POR15 onto loose rust scale will never take the place of mindful restoration with diligent preparation.
I also agree that POR15 is hard as nails. For this reason one needs to avoid build up especially in places where engineers intended ductile metal to be present - not rock hard polymers.
In the restoration world we need to work with all the technologies at hand. Non-Acid solutions like EvapoRust allow us to remove the majority of the iron-oxide from a rare part - without etching away good metal in an acid bath (as part of the prep). A thin coating of P0R15 provides rock hard water-proofing and sealing of missed rust or flash rust that appeared after the cleanup/prep step. A prep coat of epoxy based primer further readies the part for top coating . . .
In my mind it is not either/or . . . it sometimes one . . . sometimes the other . . . sometimes both . . .
regards,
stock49
