You've totally lost me tankie........
There are small steel finishers that dip steel components in an acid bath that only affects any surface material. It is neutralised and then bathed in a chemical that puts a deep black chemically induced finish onto the steel. Wouldn't stand too much rolling around in the back of your old Bedford or FV-432 but does look very good. Saw a No32 and 42 done like it and it looked good..........