Why paint it? Rock-guard it instead .. it will last alot longer
But seriously ...
Best thing to do is to pull your grill off, visit a sand-blaster and have it bead-blasted smooth. Wash it down, smooth it out (if required) with a thin coat of body filler .. prime it, paint it. Wet sand it down, repaint. Wet sand it down, repaint. Once you are happy with that, a couple coats of clear topcoat and you are ready to put it back together ..
If you have never done body work before, see if you can "volunteer" for a couple of weekends at a body shop .. many body shops would be happy to have a second set of hands available that they don't have to pay.