I'm guessing that you are either casting or moulding with epoxy.Since its one of the best adhesives there is,you would be correct in assuming the surfaces to which you don't wish to remain attached will require a covering to ensure no adhesion.You can use MDF and it is best to give it a coat of sanding sealer,denib and then apply a few coats of a silicone free wax.Other types of finish may,or may not work,so try a test piece first.For basic flat panels you can use melamine faced chipboard and add wax with the benefit of automatically getting a good finish.