I'm very familiar with POR-15, and it is a great preservative paint. The one thing that you have to do to make it work properly is to start with clean metal - that is, with no rust, grease, corrosion, or loose paint. It has to be either sandblasted, or scrubbed thoroughly with a wire brush. Anything left on the surface will interfere with proper adhesion. Unless you have a really small area to paint, or don't mind a fair amount of cleaning, I would agree with previous post and use Rust-Oleum or some other paint that can cover rust or scale. It is also much cheaper than POR-15.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.