Urethane resin is sensitive to moisture so your plaster or stone mold should be as dry as possible
then use a clear coat spray, put it on in light coats let it dry then it should be waxed as the final release.
I have been using smooth-on resin for several years and most of them are not ideal for slush casting
because they kick to fast depending on how much you have mixed, There are several resins in the smooth-on line that have longer working times. And they start out thin water like to creamy but when they start to set up they turn into a rubbery mass that will not spread it self evenly in the mold and as it sets then you have
one area that is thicker than the rest of the item you were slush casting.
I have found one resin that is great for slush casting and it is also made for coating Styrofoam
It's like latex paint it has a seven minute cure time
depending on room temperature and volume mixed
and it clings to silicone mold walls very well not like the water thin resins
Unlike some of the thinner water like resin that starts out thin and once they kick they turn into a slime ball
and you end up with one big thick spot in your slush cast pice
you can get that resin here. its called ureShell
http://www.demandhotwire.com/coatings02.html
I do prefer to use the above resin with fiberglass cloth to reinforce it
Just wanted to add my two cents
Spartan 648