I have never had to top coat POR 15. If you follow the process strictly it should work out alright without a topcoat. Treated the chassis on my A110 and Fuego Turbo and both are still good except if you get brake fluid on it like what happened to the left hand lower control arm of my A110. Brake fluid is dynamite even with POR 15.
I would cover anything under the chassis in some of that bitumen coat they sell (over the POR15) simply because stones and other hard debris chip anything that is not soft.