Lightly sand and/or 3M pad, wipe it down with Aceton. I've had good results with Grill paints (paint for the outdoor grill). Usually half flat and a nice finish. The first coat has to be light, too heavy and it will fish eye on you. Trying to paint over a fish eye is a waste of time, the paint gets thicker and the fish eye is still there.
Primers for galvanized metal, aluminum, stainless or chrome, often work well on plastics. But read the label, some will specify they can be used on plastic, some warn against it. Rustolium rusty metal primer is good stuff, it just has a long drying time. Krylon primer is good stuff also, wet sands OK, adheres well.
I've had good luck with Rustolium outdoor grill paint and spray Hammerite. The Hammerite chrome and aluminim primer is good on plastics (dries fast), the Hammerite (finish) paint is often hard to get a really nice finish on, but is usually on there to stay.
I've used the outdoor grill paint as a primer, works well, but you have to let it dry for a looong time (weeks or months), before you try the top coat. I've never tried a tack coat of grill paint and then a top coat of gloss, when the first coat tacks(15-30 minutes). It may work out, but I doubt it.
I painted a plastic chrome grill (the plastic chrome looked pretty bad) out of an 87, for my 88 (busted mine) about 4 years ago, with Rustolium outdoor grill paint, it still looks good. The grill paint seems to have just enough gloss properties, so it doesn't stain like regular flat paint does. They often call the half flat, a satin finish.
My chrome tow hooks, trailer hitch and some other stuff got a coat of hammerite chrome and aluminum primer and then a coat of Hammerite flat. My tow strap hasn't worn through the paint yet and it's still on there after three years.