POR 15 is amazing. lots of warnings and stuff, but it goes a long way and yes, you won't get it off. I have tried 36 grit sandpaper on a drill...no go. But it does stop rust dead - forever. won't wear off so it is good for under teh car, frame, floorpan, etc.

they sell stuff you can use more like primer for painting, but i have not tried it - yet.