I have these 1 x 2 diamond flat files (attached to a plastic handle) that I got from Brownells. These were a good price and in 250, 400 and 600 grit. I also use a 250 and a 600 grit set of diamond needle files. I find that the wet/dry autobody paper backed by a paint stir stick is great for finishing a metal part. I do cut the stick to various shapes for strange shaped areas.
I use a small dremel tool with a thick felt wheel and emery rouge for overall polish on small parts. This is done once very quickly so as not to have ripples. I then cut the finish back to a 600 cut to reveal any low marks or scratches I may have missed. Then the felt wheel is used again.
This lock will be rust blued, after engraving, and then the solution will be used to remove the blue and get the "French Grey" I am after. The barrel will be done the same way.
The polishing took me about 40-50 hours to complete. I am sure this is quite slow compared to faster workers, but I am fairly fussy about what I want in the end and tenaciously patient with this to get it.
