CountRW:
Nice job. I particularly like the shading you've managed on the running gear. Looks very much like a track that has run through the muck and mud of a winter's day in a European forrest.
I think the gritty texture could add "character" on a KV or a T-34, but I agree with others that it is a bit out of sorts with these StuGs. If you say this was a by-product of your choice of primer, I would recommend a change. I have settled on Floquil as my prefered primer, and feel it has given very good results. I don't think I've ever seen this kind of effect with Floquil.
However, I have seen a very similar effect with Testor's Model Master spray paints, which I often use for base coating. Others here have suggested that the effect could be related to spraying at a distance in hot sunshine, a process which causes some of the droplets to dry in mid-air. I haven't been able to check the correlation, but I always do spray outdoors, tend to hold my can some distance from the models (always more than 12, usually 18 to 24 inches), and I often paint on hot afternoons, and I do occasionally see the phenominon, so it seems a reasonable enough possibility to me.
As to the ballast/pebbles/sand... If you happen to have a household like mine (full of family), you also might try this trick: Apply a sugar sifter to stealings from the sand box. We have kids, but they are a bit old for a sandbox anymore. But the sand is still there, used only by the neighborhood cats

. Two buckets and a sugar sifter (used for sprinkling powdered sugar) allows me to "mine" for fine grain and course grain sand. Costs me nothing except 10 or 15 minutes of my time.