When rusting, you basically bring the metal back to about a 320 grit, no matter how fine you polished in the first place. I've taken barrels to the 600 grit level, and then tried to rust lightly, using diluted agent, short rusting times etc., but it always seems to come back to something around that 320 finish. This opinion is based on comparison with other guns I've done, using differing methods.
I can see potential for a slightly finer grain, in perhaps the 400 grit area, if one stops before the metal has turned a deep black, as I've seen this while working. It's rare that the color is completely even, at that point, and usually a trip to examine it in the sunlight has dispelled my urge to stop there.
You can, however, rust to a more coarse finish than 320 by rusting too long...
This is all simply my opinion on the subject. At one point I was trying to acheive a "Belgian Blue" through hand blacking. I found that some metals will allow you to stop at a slate color, which is quite pretty, but it's not blue. I've come to believe that the difference lies more in the metal, than in the process. I've used and experimented with several rusting agents. It seems to me that different agents can yeild different browns, but once boiled, it's the metal that determines how deep a black you can acheive before things start to frost. A telltale sign is when you first see small areas that no longer turn black when boiled. If pushed beyond this point, most metals will start to take on a coarser appearence. What I refer to as frost. I've found I can beat this rule, slightly, by very light rusting and careful attention, but not for long.
I think we all need to come to the realization, those doing this work as well as those having it done, that we are all at the mercy of the materials we have to work with. Which is largely an unknown. Often I have changed procedure or agent in the middle of a job, in order to try and acheive the "look" I was after. At times this worked well, others I wound up starting from scratch again. In a couple of situations, I simply had to settle for the best the given metal would allow. A German target pistol I blacked recently comes to mind. No matter what I did, or what solution at what strength I tried, after the second or third application, the metal would show an irregular grain. I even tried rusting heavily, in order to hide this in the "frost", no go. On the fourth attempt, starting from scratch, I rusted lightly and often, until I got a deep black, and ignored that grain. The customer was quite happy. I was not, and charged him as though I had only done the complete process once. I chaulked it up to a time consuming lesson and experience.
I'll probably never see another pistol like that, but I'm certain I'll see that metal again, at some point.
Jim
