The sole resin I've used for all my heads has been Smooth-On's Smooth-Cast 65D. I'm going to continue using this resin for the initial coating inside our molds. However I will be coating the insides of the heads with a more stiff product which is resistant to heat. This will help to lessen the chances of warping of our casts during shipping to hot locations. Something that happens rarely, but the hope is to not have this happen at all. The look won't change, but the strength will.