All are finely ground metallic particles in a thin shellac base. Tamiya Silver Leaf might do it too, but have never tested it for this job.

Lots of folks like Zinsser B-I-N, which needs to be shaken like a Margarita, but it thins with plain denatured alcohol and blocks virtually everything.

I've heard but not verified that the better acrylic flat white primers are quite good, too. They are easy to airbrush with the right prep, and pull down tight when dry.

MCW makes a rattle can medium gray sealer that is excellent if you have time to wait for the can to hit your doorstep.

NONE of the above are a surfacer, so you need to have all your bodywork sorted first.