You can do an awful lot of work with the palm sander. I used 60 grit to strip the heavy rust. Then, I switched to 180 grit to take it to bare metal.
The metal wheel CAN do a lot of damage to the surface of the metal (I learned this the hard way). I really like the Scotch-Brite wheels that you can attach to a standard 3/8" drill to strip metal. If you have an electric drill, I'd recommend you try one out. The pads are only about $5 and they are much, much faster than the palm sander.
