I was like eleven when I did this so don't judge me too hard, but I used a little electric sheet sander, a sanding block, and a wire brush. I also used the paper with my hands in the really tight spots. I used heavy and medium grit dry paper for better prep finishing. Once the rust was gone, I cleaned it up with gasoline, and once it was dry and free of the various particulates, I undercoated it. I'd probably use a wire wheel for the sake of expediency now that I know more, but that hillbilly method worked out fine. The undercoating is relatively thin so, like Christian mentioned, you can see rust popping up under it if something went wrong. I did the wheel wells in my Jimmy with it, and rust popped up soon after so I redid it, and that did the trick.