Well I did it with the idea that I'm going to be getting a new grill within the next few months so I didn't focus too much on durability, was more interested in seeing what it would look like. Being as its ABS plastic with chrome finish and the fact that its on the front of a truck sitting at a 90 degree angle, its likely to get hit with a lot of sand and rocks. Paint doesn't like to stick to ABS chrome to begin with and add in the factor i stated in the last sentence, it'll be pretty hard to keep it looking perfect forever. But that being said if you want to have a go at it I'd recommend the following surface prep steps:
Take grille off the truck
Tape off the area that you would like to keep chrome( I suggest a very thin tape to obtain the curves on the vertical bars of the grille, then use a thick tape to fill in the rest of it)
Sand with a 400 grit sandpaper
Clean well with mineral spirits(paint thinner)
Wash with soap and water
Rinse with just water
Make sure the surface is completely dry
Spray an adhesion promoter primer such as
http://www.amazon.com/VHT-SP440-Adhesion-Promotor-Clear/dp/B003BGSJGC
Then spray a few coats of white
Spray a few coats of clear
Wait a few hours then carefully pull off masking tape
Then let it dry for a couple days to make sure you don't mess up your beautiful paint job Mount it back on the truck