Thanks for the comments guys
More updates a comin' dont worry Tony
Robert .... the dust is bicarb of soda, i paint the area its needed to go with slightly thinned PVA then put some bicarb in my hand and blow it onto the model, when dry i use acrylic washes in black and browns to colour the "dust" the dried bicarb just soaks up the colour and when dried again gives a decent effect methinks.
All the "dust" and "muck" on the vehicles is bicarb, i'll sprinkle or blow it onto the wet washes i use and then wait till dry to add extra colour, if its too thick (or in an area you dont need) you can rub it off with a stiff nylon brush.
If you get a red/brown mix of colour the bicarb idea is perfect for doing rust also.