Many areas of the locomotive accumulate dirt (says he, stating the obvious), and one way of representing this is to make use of a suitably coloured wash that will be attracted to corners and edges by capillary action. There is a vast quantity of washes available from many manufacturers, but I have been using MIG Productions Dark Wash for several years, still the same bottle as when I first started this weathering lark. I apply it by wetting the brush (a rigger) with white spirit and then dipping it in the wash bottle so that the fluid reaches up to the ferrule. The shape and length of the bristles allow plenty of fluid to be held and to then be deposited with accuracy.
By touching the tip of the brush to a corner of detail, the wash runs into all the crooks and nannies. In the picture below you have a first-hand view of what happens when you lose concentration . . . . . .
I will leave the stray wash application where it is and it will contribute to the random dirt effect yet to be described.