If it's just surface rust, I've used Naval Jelly to take it off. I usually apply it on part, let stand 10 minutes or so, then I take a piece of steel wool and rub it in a circular motion to help loosen any stubborn places, then rince with water. Dry panal good, ready for body work or paint.
You may want to use a sand blaster around the seams if you have rust there, but would not reccommend use over entire door. It can warp metal if not careful.
