Prime the molds? Or do you meant the pieces you cast?

Please, please tell me you didn't paint your mold! :p

I never prime any of my casts. I just slop a thin base coat of black on, and do all of my painting on top of it. Which I guess is technically priming it, but I think of priming as 'spraying on a coat of a solid color to allow paint to adhere', when all I am really doing id laying down a base coat as part of the paint scheme.

I've also never sealed anything, but it won't hurt it if you do. I'm primarily just too lazy to do it. I've also been working on the theory that the paint absorbs into the plaster pretty well (as opposed to sitting on top of a lead mini), so as long as the plaster doesn't chip, the coat of paint should be fine. And I've only had one building chip, and that was made from plaster of paris. Every other building I have is Merlin's Magic, and those are solid.