Originally Posted by Sir 51D3WAYS

1.Just make sure the black stuff on the piston isn't an actual hole/divot, that would indicate that the piston is about to let go pretty soon.
2. 500 grit is a little too coarse for most RC purposes, unless if the buildup is really heavy. I would switch to 1000 as soon as I got the worst of it off. Next time try at least a 1000, doesn't matter if you clog up a few sheets, sandpaper is dirt cheap anyway, plus the scratches that you leave will be super fine and almost invisible to the naked eye.
3. You should use a plastic scraper to scrape off the remnants of the gasket, using sealant/gasket removal chemical if necessary. This chemical is usually nasty stuff, so wear PPE as appropriate. A metal blade/scraper will scratch the head, creating grooves that would allow fluids to leak past the gasket.
4. Carbon buildup on a piston is generally bad news, even if it raises compression ratio. The carbon will (usually) build up unevenly, causing random hotspots around the piston/combustion chamber, which in turn causes pre-ignition and subsequent detonation, which will put a hole in your piston.... This is not the same as your usual blackening/browning of your piston associated with normal use....