Originally Posted by JAMMINKRAZY

A dremmel with plastic wire brush and some ARO will do the trick for the most part. If it is real thick, you may need to start with a brass wire brush.

As for cleaning vs. not cleaning, I always clean my pistons up periodically. I like a clean engine, and have been pretty lucky with engine life so I will continue to do so. Only clean the top, not the sides. If you don't clean the piston up you'll eventually get a thick enough coat of carbon(especially if you're using fuels with stupid dyes in them) that it can cause some issues.