Visiting the Severn Valley Railway last week, I had the chance to examine Stanier Mogul 42968 as I waited to board the train.
I'm pleased to say that it was dirty. Not the thick filth of late BR days but a workaday grubbiness that seems appropriate.
The smokebox particularly appealed - look at the heat blisters going on with the paint. If I produced a model that looked like this, people would suggest I need a better airbrush or at least to learn how to use the one I have.
I suspect that the easiest way to reproduce this effect would be to brush some Mek Pak on the paint and watch it crinkle. Then spray another coat to seal the effect in.
What you really can see is that the boiler black and smokebox blacks are very different colours. There's some nice browniness going on by the footplate too - a bit of weathering powder there I think. The footplate is matt and dusty too so a bit more powder required.