Working in an closet in my house built in 1950. Had to cut open the wall, and found that I don't have plaster and lathe as expected. Instead, I found what looks like thin drywall (blue board) with a coating (a little less than a quarter inch) of gray stuff and then a very thin layer of white stuff (plaster skim coat?). Anyway, my question is about the gray stuff. Is that a layer of plaster over the blue board and then the white stuff is a plaster finish coat?