The Initial Griming
This is how things now stand:
I've used a combination of Raw Umber, Burnt Umber, Black and White oils for the body grime. This has mostly been acheived by almost drybrushing the colour on, before running over it with a small flat brush, moistened with a screenwash solution. After this stage, when still damp, a dry number 12 flat brush was flicked over the surface in a top to bottom, vertical, motion to remove more paint and give the rain streaked appearance.
The oils have also been used on the nose grilles and to recreate the grime deposits that gather in the cab door recesses. I'm going for subtle on this one, working from a pic that I can't post, unfortunately.
The underframe has just had it's first coat, a mix of Vallejo Flat Earth and Black acrylics, applied by brush. No airbrush has been ued on this so far (except for the repaint). It's a bit glossy still, but later processes will tone this down to dead flat where required. I did try to use the oils straight on the underframe, but it didn't really work, they need to be used on top of something else for full effect.
That's it for the time being, as everything will now need to be left for at least 24 hours to dry - the biggest disadvantage of using the oils. More as it happens