Most case color guys do in fact (hand) polish the annealed steel with sandpaper, and the standard is 320 grit. 400 would be considered a bit on the fine side. I have read that it is better to polish to 320 grit because it provides some "tooth" for the carbon to adhere to. That is also the polishing grit that Dr. Gaddy advised in his DGJ articles. Of course, the amount of polishing will determine how much metal is removed, more so than the grit used. I have tried polishing the work to both 320 and 400 final grit and found no discernible difference in the colors achieved. I can also attest to the fact that the micro scratches left by 320 grit paper in the steel are completely filled in and obscured during the case coloring process.
|