i am a PPG certified painter.
when painting plastic the most important part is to identify the type of plastic you are painting.
ABS type plastic doe NOT require adhesion promoter. TPO type plastics DOES require promoter.
fastest way to identify the type is cut a small sliver off the part. then place it in a cup of water. force the plastic to the bottom of the water several times to make sure you break surface tension floating the plastic.
IF the plastic floats you need adhesion promoter. if it sinks no promoter is required.

if promoter is required LIGHTLY scuff the plastic with a fine grit scotch brite. i use a gray.. DON'T heavily scuff it, use a RED scotch brite pad.
the promoter will swell up the scuff scratches making a mess.

plastic needs to be CLEAN before painting. i found using white wall cleaner and a gray scotch brite cleans it really good. make sure you completely rinse off the soap before painting.
you might try wiping it down with rubbing alcohol works great if on a budget.
PPG makes a 3 step clean, wipe, and prep system. it's somewhat $$$ but works great on TPO plastic.
PPG also makes a alcohol based plastic cleaner. DX103. i think it's only sold by the gallon so it's somewhat $$ also.