I had the same idea for basing, but the monochrome look of real ash made the bases look unfinished. I added some cat sand doused in glue and some splotches of thin sepia color to mix it up. The base effect was made with a mix of hobby cement, acrylic paint, a smidgeon of PVA glue, and plenty of water. Check out my blog if you’d like to see:
As for priming, you may know already, but just to be sure: make sure your rattlecan isn’t too cold when in use - run some hot tap water over it for a few minutes or put it on the radiator or something. If it’s too cold or the air is too humid, you may get a splotchy, dusty result.