Tips on painting plaster walls
Old plaster walls can give all kinds of problems during a repainting project; it is just the nature of the plaster and the plastering methods that used to be used in home construction.
Plaster was spread wet over wood lathe, for a surface that was much rougher, with more ridges and valleys, than today's nice, flat drywall. Also, unlike drywall, plaster as a tendency to pull away from walls with time, to crumble, and to continue cracking once it starts--all of which has to be dealt with before and during your painting project.
Prepare the Wall
Getting the wall as smooth and as free of imperfections as possible now will save you fits later. Secure the wall by putting drywall screws into any spots where the plaster moves when you push on it, sinking the screw heads enough that you can plaster over them. Use a paint scraper to scrape off any high areas of plaster or old paint globs. Use patching compound to fill holes and cracks, applying it with a drywall knife and sanding it smooth after it dries. For very rough, scarred walls, go over the whole wall with a very thin layer of drywall compound, using a wide knife and sand it all smooth.
Prime the Wall
Using a good sealing primer is especially important on old plaster walls because it helps hide not just colour variations but also texture variations in the surface. Once you put one coat of primer on, the white colour may highlight surface problems like cracks and ridges that you did not see before. If so, scrape, plaster and sand them as needed, and then apply another coat of primer.
Keep the texture and condition of the wall in mind when choosing a paint. Glossy paints will show ridges and cracks more than flat paints. Darker colours are better than lighter colours when it comes to muting imperfections. If the wall is really ridged and ugly despite all your efforts, consider a textured paint, up to and including heavy stucco, which is great for hiding wall problems.