I've used the black POR 15 on the underside and inside floor pan of a surface rusty Italian (sorry) car. Used the glossy stuff inside. That was several years ago and it still looks shiney as if it's wet. You have to follow their directions and use it fresh. If the can is open it "changes" and doesn't seem to work the same. If you get some on the rim of the can and pound the lid on, after it dries the only thing that will get it back off is a saw! Stains your skin big time too.
Dennis