Perry,
I use Dupli-Color high-build gray primer from the auto supply stores. It's lacquer based and I've used it over Minnwax Polycrylic (water based) with good results. Fills the sand paper pretty quickly, so I have better luck wet sanding. I use Windex to keep the sand paper clean. NOTE, I do not use dope, only lacquer or enamel for a top coat.
Paul