Use this sealer instead of any primer! Why use a gray primer when your surface should be almost perfectly smooth anyways?? Primer is fine if you have serious scratches to fill. But these are model airplanes not real cars, we should try to keep weight to a minimum.
So here is the data:
The Sealer comes in 3 colors

NCS2003 = White
NCS2004 = Gray
NCS2005 = Black

It is a Deltron product that can be tinted with concept DMC mixing bases. I know it is intended for the final step but it has some good filling qualities to it as well. I use it instead of primer. It works for me. If I find defects in the sealer coat I sand the part with 600 grit paper. (I used only 1/4 of a sheet of sand paper to do a whole plane) The stuff sands easy with no loading of the paper. Then I give it another light coat and sand again. then paint.
This stuff is relatively new on the market, it just came out late last year I believe.

Hey Mr. Redman, this is the stuff I was telling you about when I was out there in the desert.

Try it I'm sure you will like it.
Dustin