Just how lumpy are these walls? There are blades made that will scrape the lumps off the wall The plaster will probably scrape easily after you get through the paint. The walls can be smoothed out with new finish plaster or you can use a setting type of drywall joint compound, something like Durabond or better E-Z sand. It may take a couple of coats. Will you let us know what you do and how it works?