I learned a hard lesson with ceramics. You know, the part where they're really fragile and really expensive? *sigh*
Here's what you do if simple cleaning doesn't work. Go buy a replacement for $1 (that's for any size, any kind of shield) from avidrc.com and throw away the old one. Motor brushes cost more, and they don't last nearly as long as bearings, and people don't hesitate to toss those. Life is too short to spend all day trying to save a $1 bearing.