Actually what all these processes involve is an oxidized finish on the boiler jacket, not paint. Such an oxidized jacket finish is what is planned for 168.
As someone who is familiar with these various processes through the finishing of fine and antique firearms, I would offer the following. The technique used at the time probably involved a sort of rust-bluing. By numbering the steps you use, you can range your finish from a light steel gray to blue-gray to blue black to an almost jet black. It all depends on how many times you repeat the steps. The more repetitions, the blacker it gets. Surface prep is important as to how “shiny” your final product is— anything from quite matte to a very reflective shiny finish. In more modern times, new procedures have been instituted to make “bluing” more efficient and will result in a very satisfactory dark blue-black finish with considerably fewer steps and less effort. But, many of the principles remain the same— that surface prep is the key to obtaining a reflective finish. I suspect this modern bluing procedure is what is being done on 168’s jacket and the jacket will be a blue-black finish.
Is it historically correct? Actually I think it is. As I understand it, the finish was used to prevent rust and resist heat before more heat resistant paints were developed. But one thing we know is that it will make for a striking and beautiful locomotive. Can’t wait to see it finished and running!!
Bill M.