I've used the stuff many time. My Willys is pretty much painted with the stuff

It is UV sensitive, so if it's in an area that gets any sun light, you'd want to top coat it. A few things:
1) A little bit of POR-15 goes a long way. Unless you're painting the whole truck with it, you don't need a gallon.
2) POR doesn't stick too well to painted surfaces. If you are painting over anything that still has a glossy finish, you'll want to rough it up first with some sand paper or something.
3) I'd recommend using one of the POR topcoats. If you're using regular paint, you'd want to use the POR self etching primer before you paint it.
4) Don't get the stuff on you. It doesn't come off. Don't even try to get it off unless you plan to remove layers of skin.