You don't want to use a Dremel for polishing; runs WAY too fast for a good job. Using a hand method of finishing will work for a smooth surface--just go slow and use light pressure.
I mentioned this already--but if you want the best possible finish or re-finish on a Smith and Wesson handgun, send it back to the factory.
Finishing packages for your pistol can cost as little as $110.00 from the factory. Take a look:
And they do an excellent job.