If I'm not mistaken the barrier coat is epoxy with a special additive. West System does sell this additive I know because I have some and bought it locally. I don't see why putting it on first would be any better. The main thing is to make sure the cast iron is free from rust and other contaminates. Plus don't make the cast iron to smooth. If the surface is to smooth it won't adhere as good, because this is a mechanical bond.