I use Ospho and it works great. You still have to manually sand, its not a miracle product. Once you get it as clean as you can by sanding I spray on Ospho and scrub with it. Wipe it off before it drys. I had my brothers challenger completely bare metal and treated with Ospho. It stayed 100% rust free in my garage for over 5 years. It will rust if the oil from your hand gets on it or water.
I usually sand the panel right before primering but supposedly you can paint right over it. I would rather not take the chance.