I am very surprised that no answers have been forthcoming to your question. As far as I am aware the reason for being told that not going further than 400 grit paper stems from the not too distant past. Before the faceless ones in the health and safety organisations started to carve themselves a position in society and demonizing such things as Lead Mercury and a lot of the more reactive Acids suggesting that they should be taken out of the hands of thee general public, or if not that making them very difficult to obtain. Before this most rusting solutions for bluing and browning had a generous amount of Mercuric Chloride to get rusting off to a fast start, this had the potential of causing slight surface pitting if not removed in time and in doing so it made it rather difficult but not imposable to get a very high gloss surface blue so why polish the metal to a high finish. With the lowering if not complete removal of Mercuric Chloride and the addition of weak Acids that did not have the potential of surface pitting of the Mercuric Chloride also rusting formulas have been introduced containing no or very little Acid so enabling much higher gloss blue gloss surface finishes, so it is now possible to polish the metals surface to a high gloss and still have that after bluing or browning with modern rusting formulas.
