I have some but haven't had a reason to use it yet. I know a forum member has used it on a modern engine to blacken the tires. Pretty sure he immersed the whole mechanisim in the solution. Hopefully he will post his experience.
If you are doing sintered post war type wheels, common gun blue does a good job and only has to be brushed on. I have had good results with gun blue on wheels and couplers which are iron. Not so good on die cast or other metals.
Pete