Well, post 300.
I really don't think this is so much a paint issue, lots of acrylics can be used without primer now, i've used a few myself, so much as a resin issue. I think polystone is a resin mix. Resins need to be cleaned really well for the paint to adhere or the oils in the resins start to flake the paints off. I 've painted a few resin pieces and this was a problem with the earlier ones I did. They had to be stripped, washed in a strong detergent, then repainted. I hope it doesn't come to that. i love my Angel bust...hands down one of the best pieces I own, and I would hate to see it deteriorate the way some of the pics have shown. She is hollow, but at her size, I'd expect that, she would be hard to put on a shelf otherwise, so I appreciate that feature.