[Kurt in NJ]

Sandpaper can leave grit that can get lodged in the ring groove, wire brushes can leave hardened wire pieces to get jammed in the ring and score the cyl walls, a putty knife is ok, but the real tool is a carbon scraper --they are formed like a putty knife, but made of a row of heavy wires (about 1/16 dia)with an adjustable guide so the wires can be held flat for flat surfaces, or slid back so the wires can conform to curved surfaces.

after you scrape it turn the engine some and clean the cyl walls to remove what falls between the piston and the wall, also turn the engine so the valves are closed near where you scrape --there is no great need to scrape the valves thenselves.