hmm, the grain filler version sanded nicely with 400. I'd suggest that. If you are still having adhesion issues on your test pieces, I'd suggest spraying (you can get it in spray can) plastic adhesion primer. It's used on raw plastic bumpers and trim to make paint stick to it.Cleanliness is the key. Make sure you clean the sanding dust from the solarrez of firstf. Alcohol works well and shouldn't effect cured polyester.