So by bringing the Ph way down and brushing, are you trying to remove a "film" or a "coating"? I remember that many years ago, my brother somehow managed to scatter granulated lawn fertilizer into his pool that had iron in it. Needless to say, the iron stained the plaster. The thing that finally did the trick was extremely fine grit Wet or Dry Sandpaper, something like 400 - 600 grit if I remember right. It would be a difficult job but you might just try a small inconspicious area to see how it works.