Before going any further, I cannot stress enough that you DO NOT use large buffing wheels to polish operating surfaces in the lock!! I've seen WAY too many folks who did that and dished out important flat or curved surfaces and that either ruined the parts or added many hours to fix them. One proud owner who did that to a large military percussion lock actually made almost every part in the lock unserviceable.
When polishing by hand, you can't go wrong using the sort of dull brick red colored Crocus Cloth, as it would take forever to do too much metal removal that would hurt the parts.
The COARSET things I use to polish lock parts are Hard White Arkansas Stones.
Now, if I have to remove lumps, bumps, unwanted sharp edges or rough spots, then I get out the FINE or at most the Medium INDIA stones. BUT, this is not polishing, it is metal removal. When using these stones, you MUST make sure you don't cut too much metal with them.
I do use Plain or Diamond Needle Files at times, BUT this is only in spots where I need to remove a lot of metal. So, for the average home hobbyist, these are not tools to be used until you learn about metal work.
One of the jobs I enjoyed the most in my 26 year career in the Marine Corps was as the "Instructor of OJT's" (Apprenticeship Instructor) on building NM Rifles and Pistols. I really get into the weeds on explaining things, because that's how people learn the quickest and best.
I don't want to sound too much like a grumpy old curmudgeon, but at least two or three times now, I've taken quite some time to explain how to do lock work and reduce and tune trigger pulls, but after putting a few hours into the beginning of how to do it, the person I was trying to help said it was too much for them and decided to pay someone to do it.
Still, I might be interested in doing it, IF it could become one or more "stickies."
But if a person has little or no personal knowledge of working metal by hand, they are better off paying someone to do it properly.
Gus