I have something called hydrocal or something like that, that I bought for mold making, but it's kind of picky to work with and would be nearly impossible to break off once you cast into it. I think they use it for making like dental molds and stuff, it does what you're talking about where it smooths out to that glassy finish. I could try it I guess but I haven't gotten that far yet. When I originally bought it I remember reading that it does not like having hot metal poured into it and can pop and explode so I haven't tried it.