Any halfway decent painter's helper or bored painter can make paint stick to plastic. They do bumpers, mouldings, trim, interior parts etc. all the time. Chrome can be done too (BASF DE-18) but a bare plastic piece is easier. Plastic is not high tech and paint companies have been spending millions on making products that work and teaching "technicians" how to use them. DuPont showed a bunch of my emloyees a procedure involving new products (10 yrs ago) for painting the notorious (then new) Nissan rear bumper covers. At the end of this thing they had primed and painted a beach ball, then deflated it and threw the thing around. It stayed in one piece. Ten years ago. A grille is very simple. If they cant make paint stick to a chunch of plastic, you best get away from them!