Grills are "chromed" and painting it is not possible. Paint will chip off. However, if you still want to paint it consider doing:
1. Sandblast the grill, fine sandblast.
2. Use some eatching solution (caution here)
3. Primer + paint + clear coat
(all 3 available in aerosole cans)
Apply THIN coat of primer and use 1000 grid sand paper to remove nitches.
Apply very thin coat of pain and repeat few times.
Apply very thin clear coat, couple times.
Goal is to apply thin coats at the time, let dry and re-apply.
This I did on few Jeeps, BUT if hit hard paint will chip.