I am ashamed to say that I've had a project chevelle outside for about four years now. Totally bare metal quarters, doors, and filler panel behind the rear window. The metal now is as clean as the day I stripped it, and it's because I use metal prep surface conditioner, and keep it covered with a regular car cover and a shade canopy. Every now and then (months) I go and scrub it down with a scotchbrite pad and more metal prep, wipe it off, let it dry, and cover it back up. You gotta keep the rain off it though, or you are doomed. Of course, on second thought, maybe So Cal has a bit to do with it.
