I'm referring to a flat sided mould, not intricate detail. It is the reverse (wall) side of a piece of coving.
I'm using "Formula" Fine Casting Plaster. The wood is planed softwood scraps I had lying around.
Wood grain and other imperfections are acceptable on the cast plaster surface as it faces the wall. My main goal is to ensure I can release the plaster easily and re-use the mould.
I've read that plaster doesn't adhere well to wood, but I'd rather be sure. Will it be sufficient to fill any holes and sand it smooth, or would you suggest varnishing or priming the wood? What about a release agent?