One of the things I like about this finish other than being uncommon and pleasing to look at is it's real easy to do and repair. Just prepare the barrel to the finish level you want, and rub with fine steel wool saturated in phosphoric acid wearing rubber gloves. let it work for 4 hrs or overnight or so and repeat-no carding. About 4-5 times usually does it, but more is better. I don't bother neutralizing it. Then I apply oil or mink oil or whatever you prefer. If you do get a touch of rust just rub that spot with more acid on wool or re do the whole barrel. I have never had it hurt the stock finish when doing this. I have done locks this way also. It gets better looking with time I think-a nice soft pewter color. As luck would have it, I just came back from a 3 day camp with a gun I French greyed and it was shot and stored in my tent with no problems.Wish I could post pictures, but am having an argument with my computer. Phosphoric acid is cheap, but can be hard to find. I used to be used a lot to etch steel before priming prior to self etching primer for auto painting.
Turtle