i use those tiny sanding discs to clean up solder lumps on my copper tanks, and they work awesome. better than anything else i've tried.
the problem, is the discs don't last for more than a few minutes.
so, i made my own discs.
i use the wet/dry sandpaper from Ace, then put a strip of masking tape on the back.
trace circles onto them with a nickel, then cut them out with some old scissors.
then, when i put them on the dremel trip with the screw in it, i put a little polishing wheel on first so it gives it some soft support, then just push the screw through the center and tighten it on there.
they work better, last longer, and you can get a ton of them out of one piece of sandpaper.