An old plaster wall that's wavy, bumpy or scarred can ruin the look of a whole room. As long as the wall is intact, without broken or missing plaster, you can level out the bumpy texture by skim-coating, which is the process of scraping plaster over the surface with a wide drywall knife to get it flat and smooth. Using multiple layers of plaster and keeping it very thin will prevent the plaster from cracking as it dries.
Go over the wall with a belt sander to remove any loose paint, plaster and dirt.
Spread plaster into any holes or cracks, using a drywall knife. Let the plaster dry for 12 hours. Sand it lightly by hand with a drywall pad sander.
Spread a very thin coat of patching plaster over the wall with the drywall knife, starting in one upper corner and working your way out and down. Pull the edge of the knife across the surface at a steep angle to get the plaster flat and smooth. It should fill in the low spots, but be generally transparent at the high spots.
Let the plaster dry for 12 hours. Run the front of the drywall knife over the surface to take off any peaks or ridges.
Repeat the process of skim-coating the surface, putting another layer on and getting it flat and even.
Let the plaster dry. If necessary, put on a third coat. Once the surface is completely flat, lightly sand it by hand with the pad sander, just enough to get it smooth. The wall is now ready for repainting.