You should check with your paint shop, or the paint manufacturer if you are doing it yourself, to see what the spec sheets say. I have been told that flex additives have not been needed for several years, but that you don't want to add any hardener when painting plastic parts (especially bumpers). An adhesion promoter is usually needed for bare plastic, but I wonder if it would also help on powder coat.
I don't know if the powder coating on the grills has a slip agent, but if it does you will have to sand the surface lightly to remove it. I suspect it doesn't since Rob was apparently able to get good results without sanding. In any case, washing with a tetrasodium pyrophosphate solution to strip off any surface oxidation can help with parts that have been exposed to sunlight for very long.
But the most important thing is to check the paint manufacturer's spec sheets because they should give you the exact process.
Sky NA 2007 Midnight Blue
Sky RL 2008 Carbon Flash - Naked