I start out like this, but I take the master out after the plaster(I use really hard stuff) has dried and sand the surface flat, then I drill a couple keys in the surface. Next, I spay paint the surface and cavity with el cheapo enamel, while the paint is still wet, I put the master back in the cavity, the wet paint helps hold the master in place. When the paint is dry, pour the second half of the mold, the plaster wont stick to the paint. Next step is to clean the paint off with some acetone and seal, I use Elmers