Bill, I only used a slight dusting of chalk pastel, with a pale greyish-beige mixed with a reddish-brown, crushed to a fine powder, mixed together, and wiped over the model where required.
The remainder was then brushed off. The rest of the 'dust' was done by dry-brush and damp-brush methods, with the final 'look' being created by adding just a tad more than needed of Tamiya Flat Base to Future, and sprayed overall. The latter gives the slightly diffused, overall dusty look.