Ok this is something new.I thought one of the fan boys said you don't have to wash fine cast
Mold release can be difficult to remove. Fortress one of the pioneers of spun-cast resin recommend vinegar for removing mold release. You can tell if you get it all off when the resin drys flat. If it is shinny then you will have to wash it again. Try adding vinegar to dish soap and water and soak them over night. The oil will float off most of the time. I use a sponge to hold the figures under the soapy water so the release can float to the surface.
I would not recommend GW primer because of the cost. $.98 Walmart store brand flat black works fine. If it is necessary to use PVA as a binder for the primmer to stick I would not recommend the primer.