This was something another member posted in reply to a similar question(not my words):
Did you use the SEM vinyl and plastic prep spray or the SEM adhesion promoter? You should have done a bit more research on plastic and vinyl prep. Clean the part with Dawn and warm water and dry well, wipe with a good grease and wax remover with a clean lint-free cloth, spray and wipe with the SEM vinyl and plastic prep spray, spray liberally with adhesion promoter and then apply one LIGHT coat of SEM paint while the adhesion promoter is still wet. Wait fifteen minutes between coats and apply up to four coats. You can also wipe the part with a tack cloth to remove any lint or particles between steps of the prep phase stated above. Follow these steps and you'll have great results. Also, DO NOT touch the part with your bare fingers during any of the prep phases or you're asking for poor results and adhesion problems. Good luck!