A while back I got real interested in the idea of using diamond files for "rock maintenance" rather than the more traditional knapping procedure we have used. My interest was to extend the useful life of a flint.
I ordered a cheap set of diamond files online. (approx. 10 bucks for a set of 6 files of varying grades of grit or "roughness" I'm talking Wal-Mart or Harbor Freight quality which seems to be all you need for this purpose). I've found the two roughest ones to be of most use when filing French Amber flints. In practice, a flint can be filed without removing it from the lock to "touch it up" while shooting. If you do it that way, cover the lock mechanism so that the very fine, abrasive flint dust doesn't fall into your lock. (Think: surgical drapes, where only the body part being cut on is exposed). For more extensive work, I pull the flint from the lock and secure it in a small hand vise. Whichever you choose..
.let the file do the work....it doesn't take a lot of pressure. Just keep the file on the rock
evenly, beneath the edge as you draw it back and forth. As I got into using a diamond file on my flints, one of the first things I noticed was that compared to knapping I was getting a much more uniform edge so I had more flint-to-frizzen face contact area, and the flint wasn't "shortening" nearly as quickly as when I knapped them. I'll be the first to admit,
I'm not an expert knapper. Someone with good knapping skills might not see that much difference, but
filing doesn't require a lot of skill.
The flint shown here is secured in a left hand large Siler from Jim Chambers. It has fired 100 documented shots. I put a small piece of a wooden toothpick behind it as it did shorten a bit and I require my flints to be very close to the frizzen face when at half-**** for best results on the target. This particular flint went on performing well for me for a bit more. I got 140 shots out of it before it simply became too short for my tastes. It still had a good edge, and might have made someone a decent flint for a small Siler, or Queen Anne lock in a pistol.
This is my experience...yours may vary.