Tamiya's Fine Surface Primer is my choice in rattlecan, but my go-to is Alclad Primer and Microfiller using an airbrush, medium tip at 20 psi plus or minus. I prefer the white since it contrasts with grey plastic and makes blemishes easier to see and sand, but I've seen examples of using black as a base for preshading shadow detail. I'm going to try that on my next armor build.
I buy sanding supplies at a paint store that sells auto body products. In addition to very fine grades of wet & dry sandpaper they carry 4-1/2" x 5-1/2" 3M foam backed pads in finishing grades that are perfect for "polishing" the surface before and after priming. They are easily cut to size, can be used wet or dry, and last a long time. Compared to hobby prices, the paint store stuff is a real bargain.
Mike