These are by far one of the best tools a metalsmith can have for their flexshaft. They are kept a secret, somewhat, at first in art school, as instructors want their students to learn manual skills in sanding and polishing. But word gets out. I learned about these beauties after 3 years of being in school! They're magnificent for getting into hard-to-reach places, under crevices and in cracks.
These are called 3M Radial Bristle Discs, and most jeweler's suppliers have them. There are varying grits for your needs, from fire scale removal to high polishing. Have a look over at Contenti for a full description and pricing.
image via contenti.com