I'm actually a big fan of the Tractor Supply implement paint. If you add hardener it is tough as nails, cheap, and looks good. Of course, I'm not dealing with rot like the guys up north, so for me I just used some Phosphoric Acid, cleaned everything up, and then painted it on.
Epoxy primer has a strong bond to metal. Por15 gets hard and become brittle and can peel if it doesn't get a good bond. I have epoxy primer on my work trucks and on the one truck when I painted the truck black I didn't put enough paint on so when I wet sanded I took the paint to primer in spots. Since it's a work truck I left it and never got around to repainting and no rust at all. That was 4 years ago and the truck gets washed once a year (late spring). This truck survives Cleveland Ohio winters (with all our salt) and the truck looks the same as when I finished the paint job. More important is that if you sand blast the frame try to blast the inside some, wash out and treat the inside. Most frames after sandblasting will rust out from the inside out (my cj7 second rebuild was because the frame rotted out from inside out after a sandblast, ended up with a TDK frame, but the rot was a rude awakening).
I'll spray krown rust cop on everything biannualy when it's done. It won't be driven in the winter here either. It will only see 7 or 8 months of use every year. A guy I know has his 96 gmc sprayed every other year since new. Almost never washes it and zero rust. And they definitely like the salt around here.
I've used Por15 in the past but now prefer to use some of the Eastwood paint/rust prevention products on all my latest builds. Eastwood's internal frame coating is awesome stuff; I use it generously inside the frame rails and the body support channels. I use the internal frame coating nozzle to then top coat the inner frame rails and body support channels with rust encapsulator and/or chassis black paint.