"I don't recommend using any metal polishes, even fine ones like Flitz, because they ARE abrasive
The Lead Away clothes are abrasive also. That's why you can't use them on blued steel guns. To get rid of just carbon, use a carbon cutter such as KG1. For copper fouling, KG12. For lead, try Nevr Dull. You can use Flitz on nickel, but use it carefully. Once you got the nickel clean & free of scratches, keep it waxed with Renaissance Wax to make it look good & make cleanup easier. What you do not want to do is expose the nickel surface for extended periods to any cleaning agent which contains ammonia. That can raise hell with the nickel, especially if there are any imperfections which will allow it to get under the plating.
One more thing you can try is a coating of Corrosion-X on the face of the cylinder, forcing cone and topstrap. Many folks claim to have had success in making cleanup a lot easier with that technique although I have not tried it.
Again, stay away from Lead Away clothes on blue and nickel. Save that for stainless, which they work great on.