I apply as primer an acrylic white (usually Tamiya or Gunze) with my airbrush. Don't like too much spray bottles: some years ago I used Citadel white primer in spray, but that was a stinking color (my wife disputed me each time due to the odor left in the house...) and sometimes I had "blotches" in the paint.
That said, it's possible for my second Fokker that I will use grey color as primer; I'm documenting for a heavy weathering...