Grilles and trim pieces can be removed and painted separately, or they can be left in place and sprayed.
Apply thin layers of primer and paint so that clumps and drops don't form.
Plasti-Dip is certainly capable of taking a beating, but to get the most out of a Plasti-Dip coat, you'll want to apply additional products: UV protectant coatings to battle the sun's rays, a gasoline protector so you don't eat away the area around the fuel door, and so on.