I have found that Gunze Sangyo Mr. Surfacer makes a very nice primer coat. They offer three varations 1200, 1000, and 500. They act as both a base coat and a "filler". For a primer, I use 1200, thinned 50/50 with lacquer thinner. I run this mixture through my airbrush. The finish is very durable with a light grey color and sands well. By the way, I use the 500 variation as a "paintable" thin putty that fills "pits".