Im not a pro, but Ill throw in my .02 I have used the fusion and it bonds well to plastics, but its durability is lacking especially for an area abused by rocks, etc.
I prefer to use automotive paint if possible. You can get shop grade paints for about $35 a quart, clear about the same, and a good epoxy primer/sealer (for a gallon) is only around $100 and that will paint a lot of stuff with a shelf life of forever! The sealer is what adhears the paint to the substrate and helps resist chips. So for $170 you can paint everything on your truck in the same color as factory!
Another option is go to an auto paint supply store and buy some real spray etching primer, sealer, and paint. Most shops can put factory paint in a spray can too. This will be about $40 vs the $10 at walmart, but youll see the difference in looks and durability.
If you are hell bent on doing the "walmart" paint special, Plastics are not all that special, only if you plan on it flexing a lot. The ABS plastics of the grill and valance do not. Urethane bumpers are a slightly different story. I suggest cleaning the plastic very well (degreaser), scuffing, wiping with clean damp cloth, prime and paint. I suggest using the engine paint available at Checkers/oreily's with the ceramic additive and use the matching primer. This paint drys slower and allows you "flow" the paint together to help eliminate the streaking that happens with shake n shoot jobs. This paint also drys very hard and will help to resist chips as well. I have also wet sanded and polished this paint with success!
If there is a pro out there feel free to chime in with corrections.
