Bettina,
There is no question that the SS material is difficult to smooth and polish. This is partly due to the fact that it is a composite metal. The stainless component is alloy 420 and as printed, it has a high surface hardness. The bronze component is soft and abrades at a different rate than the stainless. This creates additional problems if you are trying to achieve a mirror finish. Under a microscope the partially polished surface will appear as having tiny pock marks. This is due to the uneven surface hardness. However it is possible to get a polished finish. It just takes a lot of time and careful attention. I use carbide grinding burs to remove the rough print lines(
bark) on a raw print. Then i use a progression of abrasives beginning with 180-220 grit down to 400-600 grit. At this point the surface is almost polished. If mirror bright is required now is the time to switch to polishing compunds
The absolute best and fastest finishing materials are those containing diamond. Inexpensive diamond coated grinding pins come in a variety of shapes and sizes. These will fit the chuck on your Dremel. I often use these after carbide burs. They can get into very small places. Other diamond materials come in the form of sanding sheets, made by 3M company. I believe the trade name is Micro Diamond Film. Another option would be diamond polishing pastes. These are used on small felt wheels that can also be mounted on your Dremel. These pastes range from relatively coarse 180 grit down to amazinly fine 2-3000 mesh! Overall these materials are somewhat more costly than traditional abrasives but they work better, faster, and longer than aluminum oxide, silicon carbide etc. They require a lot less hand pressure and will save you time, pain and money in the long run.
In the end, this material will never be the same as Sterling or Karat Gold. But, if you learn to work within the boundaries of the SS print medium amazing things will result. Dam the torpedoes, full speed ahead.
-G