You can use SEM black out.. comes in a spray can.. You can use a flattened single stage or a flattened clear coat if you are an over achiever and wanting to go the extra mile
We use SEM trim black for everyday cars, flattened urethane for the nice jobs. I've never had the SEM flake, though it does start to fade after a few years. Sounds like there might be an adhesion problem with your current process? I don't know.
Prep the plastic properly, shoot a sealer coat of epoxy primer, then two coats of singlestage flattened to the desired gloss- this will provide your best durability and overall lifespan but it may not be economical on a lot of jobs.