I believe that when washing demineralised water is supposed to be used
Never having been involved further than light aircraft being washed by one of the specialist contractors, I got this from an account a while ago of a preserved Lockheed Constellation owned by an enthusiast group being brought to New York to do a
PR event for TWA in their final days, and as a contribution TWA gave them a top-notch wash by their JFK crew. It was all described in great detail, including the spec of the water.