I've had issues with JF castings not wanting to hang onto primer before, as well, but I've found that if I wait about 48 hours and let it fully cure, it's no longer a problem. The primer will still want to flake off once it's dry to the touch, but after two days it's just fine. I've had that happen with other resins, too, but I've had it happen most frequently with the Flintstone castings.