POR15 has one flaw. Its extremely sensitive to sunlight. So you have to top coat it to protect it from the sun. Otherwise its bad azz at protecting your stuff. There are other brands that also work well. But I know of none that are not damaged by sun light. They all have to be top coated. However chassis black is sold everywhere so its not a big deal to top coat your stuff.
I have an '85 Chevy pickup that I'm playing the same game with right now. My frame is rust scaled up pretty good, and the rust coating process is already under way....