Plaster is one of the easiest surfaces from which to remove wallpaper paste. Plaster is a hard substance, much like cement. There is very little chance that the wall will be damaged to the point where you would have to patch up any holes or dents before you apply paint or new wallpaper. Removing wallpaper paste from plaster is a job that one person can take on by himself and complete in a day or less, depending on the size of the room.
Fill a pump sprayer with 147ml of wallpaper stripper and 1 gallon of water. These products are made just for this purpose and used properly will not damage the plaster.
Spray the paste with the solution and let it soak in for a few minutes.
Use a plastic or metal blunt edge scraper to scrape the paste off. Hold the scraper at a 45-degree angle to the wall so that the side, and not the tip, of the scraper does the work.
Repeat the process again if necessary.
Rinse with clean water and wipe the plaster with a damp sponge until all the paste is removed.
Let the wall dry for 24 to 48 hours before painting or repapering.
Wear work gloves and protective goggles when working with any type of solvent. Open the windows and keep the room well-ventilated.