Well, unless you use a rust CONVERTER or NEUTRALIZER first, it's a moot point, because the rust is still there and it's still rust. All POR15 and the like does is cover it. And I don't care what any of the marketing says, that does NOT stop rust. It only slows it down.
Well yeah. If you buy the POR15 kit like I did with the dart, it comes with a neutralizer that you brush on first. Turns the rust to a chalky white like color. Then once that dry, degreaser and then dry and then por15.