Old plaster walls can be wavy, bumpy and uneven, due either to a substandard original application of the plaster, or movement and patching later in the life of the wall. Either way, when light hits those ridges and dips, it can make the wall look pretty bad. One way to solve this problem is called "skim-coating," which is simply the application of a new layer of plaster over the top of the old plaster, in very thin coats, which fill in the low spots without building out the high spots. Make sure you are using plaster that is made for skim-coating. Your home improvement store can help you.
Sand the surface of the wall with your belt sander, just enough to take off any gloss or loose paint. The surface should be dull and clean.
Press joist compound over any cracks in the surface with a drywall knife, filling them completely and getting them flat. Let it dry overnight.
Scoop up a generous amount of compound on your drywall knife. Starting at the top of the wall, hold the long front edge of the knife flush to the wall, with the handle out at a shallow angle, and pull it over the surface, so the compound goes on in a paper-thin layer.
Work your way across and down the wall, continuing to apply the compound in a thin layer so it fills the low spots and crevices. On the high spots, it so should be so thin it's transparent. Work out any lines from the wet compound as best you can. Do the whole wall.
Let the compound dry for 12 hours. Scrape over the dried compound with your drywall knife to take off any lines or ridges.
Repeat the skim-coating process, adding another layer, keeping it paper-thin. Let it dry. If there is still any unevenness to the surface, scrape the surface lightly and add a third skim-coat.
Let the final coat dry for 24 hours. Sand it lightly by hand with a drywall sander to get it flat and smooth. Repaint the wall.