Ok, I asked the guy that I do custom work for. He is a professional model builder. He also restores real cars. You need to buy a Du Pont product called Prep Sol. You will have to buy it from a auto body paint shop. It's about $50.00 for a gallon.
He has been building resin kits for many years now. He told me that today that the quality of resin kits that a lot of casters is just worthless. He has people send him kits to build and the surface is impossible to even prep to get paint on it! He said it seems that somehow mold release is somehow absorbed in the resin part also there are a lot of fine tiny holes. He got a kit and it is rubbery and it is junk. It came from someone in Tennesee. Some bodies are very thick and a lot of work goes into opening up window openings and pillars and then after all that time consuming sanding and filling windows don't fit right.
Having a vacuum chamber and pump is a must have items to make good parts. I vacuum my resin down before I measure and mix the two parts together. Vacuuming or de airing the resin will remove moisture from it. Then I pressure cast the mold with weight on top of the mold. This helps create very thin flash around the part. I spray mold release in the molds but then I let it set for a few minutes for the release to gas out. Spraying release on the mold and making a part right away will cause parts to be rubbery and not cure correctly. There is a lot to know to make good parts.