Ive found SEM is the best if prepped right. If you dont prep correctly it wont last pretty much like anything else.
Heres some more info. http://semproducts.com/index.php/download_file/185/256/
Clean with soap and water. Clean with SEM plastic leather prep. Then either spray Sand Free or Adhesion promoter, and then color coat.
Youll need to figure out what type of plastic your spraying. Is it ABS or some sort of PP, EPDM, or TPO. Usually Ive found its stamped into the back of the panel. Sometimes not.
You can test it by using a drop of acetone. If the plastic melts from acetone then its ABS. If not then its PP, TPO, or EPDM.
For ABS use sand free before color coat. Ive used adhesion promoter instead and it doesnt work as well. Color coat will scrape off easy.
For PP, TPO, EPDM use Adhesion promoter.
If done correctly the color coat is super tough and durable.