I just use tap water,never had trouble.Walter used tap water, and German water is very hard, it didn't cause problems.Water is used only to kill the rust,turn it black, and the work is well carded afterward.Also I don't mask off areas, I just keep the solution off unwanted areas as best I can during the bluing.Afterward, I polish the flats,the sides and rear of the under lugs,rear of the barrels and front of the muzzle with worn 320 grit abrasive.I don't worry about getting water into the barrels,I just tip them to the rear to pour it out and after boiling for 15 min. the barrels are hot enough to dry instantly.If I'm not going to apply another coat right away, I don't card until right before I am.Never let the work go over night rusting, always boil it and card the next day.
Mike
