Just a couple comments.Years ago, the barrels(german)were completly tinned before ribs were attached, to prevent rust under them.Then this practice went away, likely due to cost of cleaning the excess off.If a leak indicates a rib comming loose, the bluing should stop until the rib is repaired. With regard to applying a rust preventing oil under the ribs, after bluing;I watched Walter Grass "worry" a piece of chord through a threaded hole in a bottom rib(using scribe and compressed air)and then soak the chord in Ballistol.With regard to using steam,I had a rifle barrel start turning black during rusting,because of temperature and humidity during an Alabama summertime.
Mike