I have to polish the inside of a cylinder of I.D. 20mm to (ideally) a mirro r finish - or at least something approaching it. The material is mild steel . I've got about as far as the dull matt grey you get from using fine valve grinding paste and now I need to go finer still. So I've me Dremmel ready with its cloth mop attachment and just need a steer on a suitable cutting c ompound so I can end up with the desired outcome. Any suggestions?