Hi Gerry,
Firstly, very nice work on the Tank, that's a very fine looking model there. That beer in the pics is certainly well deserved !!
As for the dirtying up bit, everyone has their own favourite techniques for this so I am sure there are a few more posts to come yet on this thread.
As you want to do sand dust, why not use the real thing and brush it on..?? All sand will leave traces of fine dust if sieved off correctly & It will stick well to Matt paint. It might even be possible to airbrush this on but that is something I have never tried so I am sure others will put me right here if I am wrong.
As for my tank, I let it sit on a shelf in a suitable place for a few months and let the dust gather naturally on it, looks great !! A very fine airbourne spray of matt varnish was all that was needed to lock the dust down without making it shiny or soaking the dust too much.
Regards........Mark.