sarah skinner has a 'how to prep your coinset before you sew it onto your base'article on her site:
www.bellydancer.info
I dipped them in warm water with a bit of detergent and vinegar, toweled them off and store them with a little sack of unboiled rice just in case. I like the tarnished loo,, but my set also has holographic sequins in some of the medallions that would be damaged by too much water.