The secret is to "over weather" with your pastels. In other words, when you reach the stage that it looks "right", keep on adding pastels until it looks like it's too much or too intense and then paint your topcoat over it. Done properly (it do take practice) the overcoat will tone down the weathering effect until it looks right
You might give a thought to applying the "dusty look" with an airbrush - a lot of RR modelers do this very effectively. Simply thin down the color you're using for the dust and spray it on the object you're weathering from a distance. Again practice is necessary to get it right. That's why I have 1/2 dozen "paint hulks" around my modeling area.