To be honest, these diamonds need keying really well, the coating on them isnt paint is a form of chrome plating, thus it needs a metal primer not a plastic primer. Here we use a very very fine compound in a sand blasting cabinet on low pressures then a metal etching primer, followed by a 2K primer, then sanded again with flatting compound before cleaning and painting.
Whenever you do a diamond you want to key all the small areas, use some scotch bright its much easier to get it keyed.....