xs hedspace,
Walter's water had enough calcium that the pot he boiled his water for coffe in had a quater inch or more around the sides and it still didn't hurt the bluing.I'm sure that the carding removed any calcium left on the barrels, but I never saw any.My water here doesn't have a lot of calcium, but it does have other minerals.Maybe carding it well and applying the next coat with steel wool did the trick.Everybody just uses whatever works for them.
Mike