i what i cant believe is why didn't the autobarn guy tell me about this.
Perhaps because he's a sales assistant not a spray painter. And if he gave you advice, would you listen to a guy from Autobarn anyway?
Make sure you get the right primer for your plastic and also the correct paint. Some will not be flexible enough and the next time you bump something it'll all crack and fall off. Also most plastics need a "pre-prime" with a solvent of some description to get rid of the plasticiser from the surface.