After you have both sand resin halves or sections completed, you need to coat both in a metal resistant mixture to help keep the liquid metal from destroying the mold during the pour. For this process i made a solution of alcohol and powdered graphite that i coated the interior of both molds with. I then used a propane torch to light both on fire. this burns the graphite into the surface of the mold creating a clean finished surface for the metal to go into. 

Note: Be careful when moving the molds and when putting them together as the sand resin mixture is extremely fragile and prone to breaking.