I use a stainless trough to boil barrels etc after each rusting, then card after each boiling. Works for me.

Also have used steam as a point-source when touching-up spots of worn or damaged blueing. It sometimes remains noticeable but always a big improvement over the damaged/worn original. Spout from a kettle is all I have ever used for touch-ups. Never worried about streaking from condensation as the whole job area gets immediately saturated by the condensing steam.
Just like boiling but not underwater!

Hope this is of interest.