Pastel chalk

Hey John,
The technique that I use is fairly simple. First select the colours of pastel that you want to use, here I usually choose earth tones anywhere from gray through brown. Then grind the pastel stick down into dust with a peice of sandpaper. Then I use a course brush called a stencil brush to scrub the chalk dust into the nooks and crannies. For an overall dusty efect, I use a fan brush and apply the pastels working from the top of the vehicle to the bottom with heavier amounts applied to the areas that would accumulate more dust, horizontal surfaces and areas that are closer to the suspension.
This is the last step that I do when finishing a model. After the dust is applied, the model can no longer be handled with the bare hand. Some people seal the pastels with a light spray of dull cote, but I don't like that finish myself.

I use a chisel point brush with short hair lengths. These are also called deer foot or doe foot brushes. They are stiff enough to get some nice psatel effects but soft enough to not cause damage or to vary brush pressure when applying pastels.

Mike

I just use a regular, cheap paint brush that I cut the bristles down to a very short length. Grind the chalk up into dust, dip brush and brush away :)
I find the effect on tires especially pleasing, as the 'dust' will gather in the crevices of the tire tread, giving a very realistic effect

What about using this product to muddy up a rally car? I guess it would work, although the desired effect is more spay than settled dust.

myounder, I don't model cars (WW2 aircraft) but I have seen some great effects using pastels mixed with a little Tamiya thinner applied to an old toothbrush and using the bristles to flick the "mud" up the sides of the car, wheel wells, mud guards, even windscreen (masking tape where the wipers would travel, flick the "mud" then peel the tape off to show cleaned wiper areas). I haven't done it but what I saw impressed me with it's realism. Hope this helps.

Cheers....Snowy.