Areas like the chimney, cab roof, firebox door, pipes and running plate will be dirty to some degree, even on a well-maintained locomotive. For this subject I have prepared these surfaces with some brush-on AMMO by mig Panel Line Wash (PLW) Black Night. This fluid is a very thin enamel paint, basically, and has a very small content of pigment compared with carrier. I have used it here to introduce a small build-up of dirt around the chimney top and base, the cab roof, the pipework and the running plate by 'sloshing it on' with soft brushes and leaving it to dry.
There's still quite a bit of work to be done . . . . . . . .