This Hornby Bulleid Q1 has been renumbered as Guildford Shed's least 
glamourous No. 33019 and is midway through my non-airbrush weathering.
I
 used to airbrush weather all the time, but now favour pigment washes and 
drybrushing, these giving me giving far more control and a slightly more random 
effect like the real thing.
In time the upper surfaces will receive a little 
matt varnish misted on from above to suggest fallen ash (Testors 
Dullcote from an aerosol), otherwise I favour the slight sheen of a 
working engine, remembering clambering 
around mucky working engines in Poland in the very early 1990's. Working
 steam has more of a warm smeared sticky Christmas Pudding look than 
pure Barry Island rust often portrayed.
I tend to weather in stages, splitting into different sessions over a few days rather than all in one go, I can them appraise the result in different light including that of the layouts it will be running on. It also allows the paints to set well between sessions.