I have also had to repair interior plaster walls that are adhered directly onto exterior masonry walls (yes, its a VERY old construction!) . If the original plaster wall is sound, but is simply chipped, scarred, lumpy or unslightly in that way, then you can skim-coat. I have had excellent results by applying a thin layer of plaster, and then covering/embedding the entire wall surface with thin, flexible nylon screening (like you use for windows). Let that dry and then put one or two thin coats of plaster over that. It has made an excellent surface and has lasted for years and years and years with no cracking or separation from the original wall.