I have done this. I recommend that you use scotch brite or a very high grite sand paper like 1200grit. Sand all areas that you can sand. Then prime wet sand and then seal with a primer sealer and then shoot with color. I have yet to have a plastic grill chip. Also I recommend to prevent chipping to put a clear coat on it as well. Plus the clear coat will give it a nice highgloss shine.
2 coats primer, 1 coat sealer, minimum of 3 coats color, and minimum 3 coats clear coat. That is my normal rule of thumb.