I think the etching soln might be too strong. I'd dilute it in half at least and try it that way. A 5% soln works well.
Also, find some way to get the entire set of bbls under running water while you card them.
You need to flush the etching solution off and away from the metal.
I'm guessing in the way you're doing the carding in a shallow tank you are just diluting it off of the surface and reapplying it again and again as you card.
Wiping them down afterwards is still leaving some etchant on the surface no matter how dilute at that point but it's still working at removing any buildup of pattern you've achieved.
So each time you're back to the beginning...
Humidity sounds low for whats usually quoted, but I'm slow rust bluing a set of Fox bbls now with Laurel Mtn and they rust fine in a 24hr cycle. Temp is kept at 63F,,Don't know what the humidity is but it can't be much. Outside it's in the 20's. I've been doing it that way for years. A much finer rust and actually a better looking finish I think.
Hang them in the bathroom after a shower w/the door closed if you want to accelerate the rusting. It'll do a fine job in about 2 hrs. Inexpensive rusting cabinet that doesn't take up any space. You can get several cycles a day done that way if needed.
Use the shower to wet card the damascus finish bbls too if you have no other way such as basement wash tubs or it's warm outside and you can use the hose..
A lot of this stuff takes an understanding spouse or living alone.
Just some thoughts.
