As has been stated, bluing does not have to be removed to brown/rust the barrel. You are just adding an extra step and wasting the BC blue remover. It is helpful to remove all oils, waxes, etc. before you brown though...including fingerprint oils.
A good even rust brown takes several applications of browning solution in a humid environment to achieve a nice finish.
I have never tried the mustard brown method and don't plan too. Vinegar is a mild acid and takes too long to work...this is not a fast process anyway, why prolong it? Suspended in mustard, I would think it would give mottled or spotty results, at best.
I haven't heard of the gauze wrap thing and I believe I have read about every longrifle building book out there....or perhaps I have let myself forget about. Anyway, in order to brown you need rust and in order to rust the metal has to be damp. So, the gauze would have to be damp....and wrapped thin so air can still get to the metal that you are trying to oxidize.
If you want a consistant, built-up, durable, rust brown finish buy Wahkon Bay, Dangler's, or any other HCL based rusting solution.
Or do an experiment and let us know the results. I would be more than curious about the time it takes to achieve a good finish....not so curious that I would do it myself though.... :grin:
Hey, isn't that BC blue remover acid based like naval jelly??? If so, that will also rust your barrel if left go. :wink:
Enjoy, J.D.