There are lots of good options. It just depends on your budget and how you want it to look.
I've never used POR15 but I know those who have and it very tough, messy, and if it gets on your skin it takes days or weeks to wear off. If exposed to daylight a lot it will turn chalky. It's very durable and also very hard to get off should you ever change your mind.
If you have a cheapo spray gun and air compressor, some cheap primer and black enamel paint from the tractor supply store will make a very nice clean looking job.
Another option is 3M rubberized undercoating in spray cans.
I'm sure somebody will chime in with more options!
