Good sealing rings will have uniform, dry carbon on the piston crown all the way to the edge and all the way around. A lot of pretty good sealing pistons have a little oil wash on the bottom.
I use mostly chop sticks. I will very carefully use a wide bladed Exacto knife on a stubborn chunk. I don't use scouring pads. There is no need to remove the thin layer of carbon, just the cake.
The only way to deal with worn ring grooves is to get new pistons. It's critical if you want to fix it. Rings seal on the cylinder wall but they also seal on the piston. A lot of people forget this fact.