You can paint straight on as long as the surface is tight - it has a rust inhibitor in it. I think I uses a quart for the frame and axles, so I guess you will need 2. Don't pour out any more than you need as it starts turning itself into plastic as soon as its out the can, and ffs don't get it on your skin. Any bits (doesn't sound like there are any) that will be exposed to the sun need to have another layer of a different paint on top, as POR15 doesn't like ultraviolet.
Dave Milne, Scotland '99 TJ 4.0 Sahara