I used Por 15 and have been very pleased with it. The downside to POR 15 is it has to be topcoated it exposed to sunlight as it is not UV resistant. This was not a big issue to me as it only took about another 30 minutes to paint over the POR 15 when it was still tacky. The POR 15 has left a very nice, durable finish that has held up great. I live down three miles of gravel road and have very, very few rock chips in my frame.......now the paint in the fender wells are another story...
Others have used another brand (Mastercoat maybe?) and liked it also. When I did my frame I was unaware of the other brand or I might have tried it. It does NOT have to be topcoated to be UV resistant.
One last thing on the POR 15. Coverage was awesome. I bought two quarts thinking I would need it to finish my frame and only used about 3/4 of one quart. The rest I used on my axles, front bumper, tow bar, a seperate tractor frame, one 16 foot trailer frame and I still have around 1/4 quart left. I keep it in the shop fridge with a piece of saran wrap between the lid and the can to help keep out air and seal it up.
Good luck!