Thank you as ever Steve, very kind.
I don’t use the apply and wipe off method on diesel roofs (or coaches and vans).
I actually used a pre-shading coat through the airbrush in a dark grey-black around seams, joints and hatches.
I then, using light layers of roof dirt (a mix, not the full ‘roof dirt’ colour as supplied by various manufacturers) through the airbrush, cover the whole roof but allowing the pre-shading to show through.
The next step was to add various weathering powder runs, that I fixed in place using the AK Pigment Fixer (airbrush) followed by further coats, thinly, of the roof dirt colour(s) again, using AK random pattern masks to get the small and random blotches etc.
Hope that helps.
Neil.