Advice on frame/under chassis painting (POR15 use)

You are not going to get much UV on the chassis. If you plan on keeping the car outside on a reflective surface, maybe. You can top coat the POR if you do it while it is tacky. Wait until it starts to cure (30 min to 2 hrs depending on temp/humidity) and spray the paint of your choice over it if you want the extra protection. I used a UV block satin clear over mine.
If it dries it is very difficult to topcoat.

- Bill