Thanks so much for the compliment BRealistic, it's much appreciated. Basically I follow Tim Shackleton's weathering techniques, but I probably spend a bit more time in what I call "finishing" the model.
By finishing I mean using powders, painting enamels on with brushes and using gouache. To me at least airbrushing is only the start, I personally don't think you can completely weather anything by just airbrushing.
On the Compound I spent probably 2 hours finishing the model of after about 45 mins worth of airbrushing. The under frame got the basic 60:40 mix of Humbrol Matt Leather and Matt Black. Always make sure to have powered track or a 9 volt battery on hand to turn the wheels, I've completely forgotten plenty of times and a 'nice' shadow mark is always left by the motion work. The same goes for tender wheels, which are also easily forgotten in the process. The loco body is weathered again with the leather and matt black, the proportions just swapped. For the smokebox itself I swap out the matt black for Humbrol 27004 Gunmetal an absolutely brilliant paint. When dry I give the smoke box a gentle rub with a cotton bud to bring out a nice shine sort of burnished effect on the smokebox. Streaking is fairly simple with a 1/2 flat brush only barely damp in white spirit (I find this the most 'gentle' of all that metho/turps family)
Powders come next. I originally used the Carrs powders but I find their Rust colours far to intense, however their greys and white are perfect for ash. Mixing up a few of the grey powders I will lightly brush some under the smokebox, where ash has been shovelled out and also after seeing it in several colour pictures, around the lubricators. I presume this is to 'mop up' any excess oil that may have been spilt in applying it.
Mig powders are next and they are just brilliant. The three main shades I use are Black Smoke, Track Dirt and Dry Mud. Black Smoke is an obvious in it's use but the other two I use as an alternative for rust, Track dirt for older darker rust and dry mud for newer rust. Application is fairly simple, I use soft brushes with as minimal powder as possible and lightly brush it on building layers and 'melting' the colours into each other. Dry mud is used on brake blocks to simulate that awful colour that builds on them, both colours are used and mixed in places like the bunker and near anywhere water has been. It is all toned down by the lightest of black smoke brushing, just to get as much sharpness out of the powders as possible, the photo I copied showed rust but it didn't stand out a great deal.
For the smokebox and top of the boiler I lightly brush on black smoke powder, then with a stiff brush, brush down, streaking the powder like the paint underneath it.
With the powders all done (hopefully) I turn to brush painting enamels. I usually begin by taking a newish tin of gunmetal metacote and patch painting handrails, grab irons edges of steps, Basically anywhere crew members come into contact with the loco, its done to show where the weathering has been disturbed, the dirt shifted and the black livery (and sometimes bare metal) shows through. That's left to dry for a little while I move to axle boxes. A much older tin of Gunmetal is used here, 'dabbed' on with a old stiff brush. I polish some axle boxes, giving the effect of spilt oil or leave it thus giving an effect of much older 'caked on' oil. Humbrol's brilliant Coal Black is sometime used to show 'fresh' spilt oil.
Humbrol gloss varnish is used to replicate water around the tender tank (inside the open tender cap I used deluxe material's 'glue n glaze') gloss varnish is used on lubricators as well.
Motion work is one of the trickier things to do in my opinion just because the colour can be a to get right. Sometimes the colour sprayed onto the wheels etc matches perfectly to the photo you are using, in that case I simply brush gloss varnish with a fair amount of thinners onto it.
For the Compound though the photo I copied had basically a different shade for each 'rod'. As a Compound only has two, I decided to chose two similar shaded ones to copy the best I could. To do this I use Humbrol number 9 and again gunmetal mixed in various proportions and brushed on again with a lot of thinners, building up the layers and (trying) to make realistic oily tones.
After that is done I will go again with either a cotton bud or a stiff brush and in a patchy way run over the areas I painted with the gunmetal to bring out that little bit of shine on the handrails etc when they are hit by light.
Then it is a simple job of going over the loco looking for things I have missed (there always is) and then she is usually all done!
This is more of a general step by step in what I do but from now on I shall be more detailed in my step by steps for each loco I do. I hope you enjoy.
Broc