Removing a silicone-based release agent from your cured urethane casting is not easy. The most effective way is to sand blast or bead blast the surface of your casting. Sand blasting, however, removes a micro thin layer of the casting's surface. If not done properly, you can damage or remove fine detail.
You can then prime and paint your casting.
If your goal is to paint your casting after demolding, read our article FAQ: How Can I Make Paint Stick To My Castings?