@ mraaker - I've drybrushed up from black primer as follows: GW Scorched Earth, GW Graveyard Earth, and GW Commando Khaki, with a final light drybrush of Bleached Bone. It doesn't have that silvery-grey sheen you're looking for, but it simulates weathered wood well enough. I guess you could substitute a grey for the Commando Khaki, and you might get the effect you're aiming for. In fact, I must try it on my next building!