Here's an extract from the PDF build log for the model:
'Flory Models' clay washes: These washes come in various shades and consist of a suspended and very fine clay pigment. They are brushed over the surface to be weathered and dry in around 30 minutes. When dry, use either a piece of good, absorbent kitchen roll or a brush used for oil paint (as the bristles are harder than normal painting brushes) to remove as much of the clay wash as you need to achieve the desired effect. Once dampened, the dried clay is re-activated and the clay wash can be removed or worked as required.
First I seal the surface with airbrushed ‘Alclad’ Klear Light Sheen (ALC-311), which dries quickly.
NOTE:A gloss coat tends to stop the clay wash ‘gripping’ the surface when it is applied and it can run off or just puddle. A matte coat can cause the clay wash to ‘grip’ too much, making it difficult to remove or even to wash it off completely.
To apply the clay wash is just a matter of brushing all over the surface to be weathered. It doesn’t matter really how much is applied as it can be left on for any period, as it is easily removed without any effect on the surface underneath. The washes I tend to use are the ‘Flory Models’ Clay Wash ’Grime’ and ’Dark Dirt’.
I use a fairly stiff oil brush to brush off the clay wash, but for smearing effects, an only very slightly damp brush or absorbent paper can be used, but even then I dab them onto a dry piece of the paper. That’s how ‘damp’ it needs to be. Any wetter and you’ll find that you are removing too much of the clay wash. If that happens you would have to re-apply the wash and start again.
That said, if you not happy with the final effect, you can easily remove the clay wash by brushing with a wet brush or even airbrush water over the surface. Dry off the surfaces washed and then re-apply the clay wash and try again until you are satisfied.
The technique is to brush over the surface to re-activate the clay wash and at the same time, to smear it over areas that had no clay wash. It’ll dry more or less straight away.
Then I’ll very lightly stiff brush and/or use a piece of damp absorbent paper or brush to remove as much as I want until I get the desired effect. If I remove too much I just reapply clay wash to that area and repeat the removal procedure.
Once finished, just run the brush under a tap to rinse out any residual clay pigments.
Finally I seal the surface with airbrushed ‘Alclad’ Clear Light Sheen (ALC-311), or other surface sealer, which will seal in the applied clay wash.
NOTE: Flory washes can be mixed to create other colour blends.
Mike that is interesting and quite complex. What I noticed and didn't see and explanation for is that in the first pic you can see what appears to be round blotches which resemble the after appearance of drops which have fully dried and left an outer rim of dark pigment. Almost looks like you had flicked the end of the brush with a finger tip to deposit splashes. Can you explain how that was achieved?