Never made a master from plaster, though I think the following would work on it too. Most of my masters have been wood. I prime it with a full bodied primer like Krylon, filling with auto glazing (spot) putty if needed (several coats of primer generally do on basswood, with sanding between coats.
I then sand the last coat with fine sandpaper (like #400). I then paint it with gloss white paint. I put auto wax over that, before pouring the RTV mold material around it. This is probably overkill, RTV resin doesn't stick that well, but it is expensive and I don't want to take any chances on messing up the mold.
I have painted plaster with Krylon primer and enamels and it works, so I think the steps would work for a plaster pattern okay.