I used a mix of 1 part 'rail white', 4 parts matt black and 5 parts BR early bauxite enamels mixed with talcum powder. I dabbed the mix on with a medium stiff brush with splayed bristles. Once it was almost dry (tacky to touch) I painted a random line around the larger patches using a slightly thinned medium rust colour. I then dry brushed downwards immediately with a large, soft brush slightly loaded with a 70/30 black white matt mix. I then added various subtle streaks and runs of fresher/lighter rust colour. For the underframe it was just a case of dry-brushing with black and earth colours along with brake dust and grime colours with a light rust on the iron brake blocks and a very dark grey (graphite like) touch around the greased points.
Although a tad blurred the effect is shown on this pic...
Regards
Stu
Al