That one was just good old wet sanding with the paper over a small wood block. Started with 220 grit and progressively worked up to 600 grit. When I do need a file, I use a diamond coated flat that's 600 on one side and 1200 on the other.
If you're going to Duracoat it, you only really need to go to 220 - 320. You want something for the paint to grip to. I tried the Duracoat "blue" and didn't care for it. Ended up going over it with a different color.
Here's a SAR that I Duracoated in their HK satin black.
If you have access to a small compressor, I highly recommend buying a $14 Harbor Freight touchup gun, and buying the DC liquid instead of the aerosol. More cost effective and gives you a better finish.