Try using engine enemel by Plasti-Cote. It is good for high temps and comes in spray cans. I use it on all my plaster molds and have made hundreds of pours without recoating. It can be purchased at Wal-Mary or any auto parts store. Cost is $3 - $4 and a can will paint several molds. It usually takes 3-4 coats with only an hour or so between coats. Have built , painted and poured in the same day!!!