From the,probably more than you want to know files,haha.
"If it's a floater,use promoter" is a time old saying in auto collision paint world.The notion is,slice a pce of plastic off the part,toss it in a glass of water.If it floats,use an adhesion promoter.
Another technique,paint plastic with an epoxy primer.... Then proceed as normal.
Another,mechanical bond by sanding....
And one of my favs..... on certain "slick" plastics,the only way to glue them is to wave a propane torch over the plastic.The specifics gets too involved for me to explain(sumthin I'm not good at anyway) but it is the only way you can glue certain varieties.Well,painting is a first cousin to glue,and the torch technique is a bonafied way to get the plastic "prepped" for paint.But there's a short window of time for the glueup or paint.....15 minutes to maybe a 1/2 hour.
If I remember,will take a pic or two of a pretty easy to make fixture/jig for holding stocks for spraying.