I last used a Carrs chemical blackening stuff many years ago, with unhappy results. I don't know if the formulation has changed since then, so can't really comment. The Birchwood stuff works a treat, and I started using it after reading Martyn Welch's book on weathering.
The surface to be blackened has to be fairly clean, or at least given a going over with a brass or steel brush. I used to apply the fluid with a cotton bud, but now I use an old sable paintbrush - I mean really old! I also dunk things like coupling hooks and buffer heads directly in the bottle for ten or more seconds, and then let them dry before giving them another brush, buff and dunk if needed. A method for blackening larger areas, such as complete loco frames, is used by Ian Middleditch. He dilutes the Birchwood chemical in quite a lot of tap water, and leaves things to "steep" over a period. I first used this method successfully on a JLTRT pannier tank build.
Once dry, you do need to give the parts a buff to remove the dried chemical. It leaves a kind of sooty residue, which is sometimes desirable for the effect of dirt, but most times needs polishing away.
but my soldering skills would allow me to create such a nice looking wagon unfortunately
