originally.
the blued parts on A3,s were not polished, only debured, and black oxide coated. original formula..sodium hydroxide, amoniom nitrate, and cyanide mixed with water, heated to temp,
that formula is not in use anymore, and the one Emeri has listed is one of the ones used today.
the phosphate coated parts were shot peaned prepped, then manganese or Zinc phosphate coated, then cured in oil, or a chromiate curing oil, then oiled and or greased before assembly.
on a rebuild.
most parts to be refinished were sandblasted. then coated in manganese phoshate, or parts replaced...with the finish applyed by the manufacture of said part.