I have found that POR15 really needs an abraded surface to stick to. A sandblasted surface is perfect. On smooth metal it will just peel right off. It even stresses this on the website
In relation to the original question
I would abrade the POR15 with some 400 grit sandpaper (your poor hands and going to be blistered) and then put a very thin layer of tie coat on. Then once that is dry put your top coat eg rubber coating
x2
needs sand or media blasting of some kind you best hire a portable blaster, buy a good breathing mask and an empty shed you can easily sweep up afterwards. Do it right first time and even to save money depending on color use Por15's Whitecote for white or with a tint from PPG for the colour you desire or Blackcote (not a fan of black myself but most are) and apply with a cheap spray gun as you will have to throw it away afterwards cause this paint is like hard chrome! or even use high quality rollers, no runs, orange peel or need for quality application its an amazing product. Try and scratch white or blackcote at a supplier that has the samples of Por15 products on railway sleepers and see what I mean, even took out the stanley knife and still had trouble cutting a scratch into the stuff as my sharp keys had no impact whatsoever.