Ed,
I used the Pilkington's at full strength and let it sit for an hour. Then I add another coat cut with water to 50% and let it sit 3 more hours. By then it has a thin layer of orange "rust", which is actually more of a discoloration than what I'd call rust, and do the boil and card. The rest of the rusting solution is applied at 50% strength and the tubes allowed to sit for 4 to 12 hours to develop an even orange film. My garage is unheated and it was raining all week, so I did not need to use a damp box this time. I previously tried calculating the strength of the etchant solution and gave up on that. This time I just added some etchant to the water until I got the results I wanted after a 5 second immersion, which was when the white strands in the damascus pattern started to visibly brighten. When the solution is at that strength, its color reminds me of Lipton's iced tea.
I etched for the first time after doing 3 boil and card cycles, when the metal had a nice even black to it. After that, I did the etch bath every other boil and card cycle. I'm not claiming to be an expert; this was only my second set of damascus barrels, and quite by accident, as I said previously. But I'm reasonably happy with the results, considering that I'm still learning the nuances. Good luck.
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