i've never tried to paint my grille, but i majored in industrial design(product design) in college and we had to actually make scale models of whatever it was we were designing at the time. soooooo, i have alot of experience painting all sorts of materials with all different types of paint.
my advice would be to get some really fine sandpaper and sand every square inch that is gonna be painted. not too hard, just scrape it up a little bit so the paint has somethin to grip to.
then wipe down everywhere you sanded and get all the dust and dirt off.
next start painting. i'd do between 2 and 4 thin coats. make sure you paint evenly and keep the paint can the same distance away from the grille as you paint.
i dont know what your grille is made off, but i'd get a paint that is made for that certain material. (i.e. plastic paint for plastic, metal paint for metal)
i'd buy the most expensive spray paint you can find, you really get what you pay for with spray paints. i used expensive paint on a shovel i designed and the paint is still on there even after 2 years of use. but just know, it's spray paint, one day it's gonna come off.
it's hard to explain all this on a post, it'd be easier to show you, but of course thats not possible.
hope this helps.