A good cream to polish the bottom of the button and the piston is a stuff called Metapolish. It comes in a tube, the same size as tooth paste and can be found at auto detailing stores and/or hardware stores. It is generally used for polishing rims and anything metal. Make sure you don't polish the side of the piston and inside the sleeve as you know that polishing is a form of grinding...so compression would be reduced. Clean it off with motor spary after you polish. You can also use this to polish the crank in certain places and the exhaust outlet.

Hope this helps.