making negative plaster mold from a positive plaster mold

[zombyfellow]

I have a positive plaster mold/copy of a body part which I created using alginate and plaster bandages. Now I would like to create a negative plaster mold by pouring plaster over the positive mold. My question is, what do i cover or spray (if anything) on the plaster positive so it wont stick to the negative that i will pour on top of it? I heard of synlube 531 mold release spray but I believe that is used on top of clay so it wont stick to polyurethane or rubber. The positive (and the negative I want to make ) is made from hydrocal (a hard plaster with a quick set time)
thanks