An old plaster ceiling that is wavy, cracked or has other problems with a surface that isn't perfectly flat doesn't necessarily have to be torn down and redone. Another option is a process called skim-coating. Plaster is scraped thinly over the surface so it fills in the holes and dips without adding significantly to the weight of the ceiling. But you first have to fill in any deep holes or cracks.
Remove all loose paint, plaster and other material from the ceiling by scraping it with a drywall knife.
Run a vibrating sander over the whole ceiling to get it dull and free of any dirt. Brush it off vigorously with a broom.
Spread wall plaster over any holes or cracks in the ceiling, using a drywall knife. If there are larger holes or cracks (wider than an inch or so), use pieces of drywall tape as backing inside the spaces before applying the plaster. Get all the repaired areas flat and smooth. Let the plaster dry for 12 hours.
Use the drywall knife to spread a thin layer of plaster over a 6- or 8-foot section of the ceiling, starting at one corner and working outward. Drag the long edge of the drywall knife over the surface, keeping the angle shallow to apply the plaster in a paper-thin layer that fills in the dips and low spots.
Let the plaster set for four to six hours. Run the drywall knife gently across the surface to remove any lines or ridges.
Apply a second layer of plaster in the same way, keeping it paper-thin. Let it dry. Lightly sand it by hand with a drywall sanding pad.
Assess the surface and add a third layer if necessary. Sand the final layer. Brush off the surface with the broom. Repaint.