Cleaning a Katadyn Vario Ceramic Cap

Is there some trick to cleaning a Vario's ceramic cap?  I use the scrubbee they provide and although it works on occasion, it just as often stops getting the cap clean and flowing again.  I know I can buy more ceramic replacements, but the frequency with which the caps fail makes me want to field (or home) clean them.

Any suggestions?  Vinegar?  Baking Powder?  Boiling?  Bleach?  Sandpaper?  Something else?

Thanks,
Steve