I'd do a gray epoxy paint, not primer, then over spray it on the out side in body color if I was looking for O.E. but really I think it's best and less time and prep to just do the bottom body color and be done with it.
Unless it's a 100% OE concourse restoration, that you want judged, I'd paint the entire underside body color. I know it's not correct, but the primer just looks like the painter didn't try. And the purists don't need to jump my comment, I know it's harder to replicate the OE look.
