Scrub it with scuff stuff, then activated ad-pro, epoxy reduced 1:1:1 then your choice of color. Being you are doing black you could just use the epoxy in the ratio I mentioned or use something else like SPI matte black, etc. The thinned epoxy versus full strength helps keep the grain visible on plastic (that has grain) and doesn't look "thick". Has a nice matte look as well. If you need a different look then experiment with epoxy or one of those trim coatings over your epoxy sealer.
Flexible stuff like the plastic/rubber around the old Fox body Mustang 1/4 windows do well with epoxy, because of the flexibility. Those things were a problem when the cars were a year or two old. Wish I had it back then. Did some for a guy about two years ago now (un-reduced epoxy over activated ad-pro) and saw it recently, they are holding up great. No cracking or peeling. Activated ad pro really helps with the adhesion.