Bill, the only rusting agent I have used is Laurel Mountain Forge. I usually wait for warmer weather with more humidity. Using their product as recommended by them, I wait until the barrels have a nice orange rust and then take out of my rusting box. I usually just wipe them down and rust again. After the second rusting and maybe a third I will boil for 5-6 minutes and then card. If the barrels look decent I will etch them to see how they look. Sometimes I can get away with 5 rustings, 2-3 etchings. I only etch for 10 seconds now as my solution is old. The more you rust/etch the darker and more pronounced the pattern will look. I don't do this because I only want the barrels to look nice not great. I have never used baking soda to stop the etching and have never had the etching solution continue to work. When the barrels look good to me I lightly heat them and apply a light coat of linseed oil to them, let them dry overnight.
I will be doing two sets of barrels soon, one is Bernard and the other Stub Twist. I will post pictures of the various stages.