Hi Paul,
Is this your first attempt using your airbrush on the 85? Did you have any practice prior to this or did you jump in at the deep end? It looks good to me.. when I first saw it, it took me back to the 80's! So much so I dug out my old photos for a comparison of your model.
I have happy memories travelling up n down the WCML behind these fine machines. School holidays permitting and limited finances my friends and I would buy our 7day Rail Rover ticket and hardly see our parents until the next week!
One thing I did notice was on the body sides you have sprayed a dusty brown quite high.. this is what prompted me to dig out my old snaps! As I recalled the hot summer of 84 I was certain I had photos of 85's covered in this dry brown dust looking just like yours. Alas I could not find it, memory starting to play tricks I guess!! I did find a photo of a class 81 which shows the dusty side I was referring too (see below)
A picture tells a thousand words as they say.. so here are a few more of mine.
As you say, diesels are much more oily than electrics.. but as you can see they do get mucky! The roof collects dirt from the overhead cables, especially after it has been raining. The 85's have a small ledge/ rebate on the body side below the windows/ vents. This ledge would collect muck, rainwater would puddle and start to run down the body creating dirty steaks. These locos would have been used on passenger work which may have helped keep them cleaner as they pass through the carriage wash plant.
I think you have made a good start on your model..as you have found airbrush alone sometimes is not enough. I find using a combination of different weathering techniques in layers like Bob states enhances locos. Powder, dry brush, washes all bring the loco to life.
Steve