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.
It was something stupid and if someone is checking out the underside of your car and they're not an OE judge then just ask them what they found so interesting and to also check a couple of muffler clamps too. 
