Plaster is much more solid and stable than drywall. It insulates well, blocks out noise and can last for well over 100 years. Even if your walls are old, cracked, or warped, it is still usually worth it to save your old plaster. You can usually even the walls out with sand paper, patch up any minor damage with spackle. If the plaster is damaged, preparing it can be time consuming, but painting it is no more difficult than painting any other wall.
Patch any holes in the wall. Fix nail holes and small cracks by smoothing spackle into them with your fingers. Follow the link below for instruction on how to fix more extensive damage.
Give any patches time to cure properly. Also, make sure that there are no wet spots in your plaster. Plaster takes time to cure.
Sand the plaster. Wrap 60 or 80-grit sandpaper around a sanding block and rub it against the surface of the plaster. Level any bulges in the wall.
Use a tack cloth to wipe the dust off the walls. Be sure to get to every area you sanded.
Paint the walls with oil-based primer. Allow at least 24 hours for it to dry.
Paint the walls with oil-based paint. You may need to use 2 or 3 coats with time for drying in-between to get a really nice coat.
If your plaster is stained or discoloured, use stain-blocking primer. Depending on what paint you use, you may have to stay out of the room for a few days after.