I have always cleaned the pistons in a similar manner. Afterwards, I spray a small amount of silicone spry into the piston seal area and let dry. It pubs the rubber parts a bit and really smooths things out.
The real problem I had was the MC seal were going bad.
old school...like polishing spokes - use a shoelace soaked in brake cleaner...pull the pads and put a piece of plywood in there ( not too thin ...not too thick..."just right"(5/8"?))pump the caliper up...wrap the shoelace around the exposed piston(s)...buff 'em clean with the lace...also clean the back of the pads while they're out...best fix is to rebuild the caliper ..the kit (from EBR) isn't too costly.