My trick for making sand/dust weathering (or to fade any type of paint) is very light coats of XF-57 Buff in the airbrush from a distance.
It mellows out the paint job and blends it together. For a desert type weathering I would just go a bit heavier on the buff!
You can also use AMMO by Mig Light Dust 1401 to add "pools" of dust in areas it can collect but you have to be very careful and only add it sparingly and then fade it out along the edges. Took some practising until I worked out how to use it properly. My little 17ml bottle will probably last me a lifetime - that's how little you should use. Often buff is enough.
You could also try a pigment scrub. Go over it with a sand pigment and really scrub it in. Then dip a brush in odourless thinners, wipe most of it off on a paper towel and try to brush off as much of the pigment as you can. It leaves a dry dusty look to the model. It's what I used on my Bradley build.