There's a limit to how much you can do for plaster walls that are uneven and wavy, or plasterboard that was poorly installed. If the situation is bad enough, as a last resort you may have to rip out the plaster and replace it with plasterboard, or retape and finish the existing plasterboard. Among less-drastic solutions is covering the wall in a low-sheen paint that has sand or other texture material, which can obscure some of the ridges and dips in the wall. However, the wall surface must be solid and unbroken before you start.
Remove the furniture from the room. Cover the floor with tarpaulins. Take off any electrical socket and switch plates, and remove other obstructions.
Scrape the wall with a paint scraper, removing all flaking paint and any loose plaster. If the walls are greasy, use a tri-sodium phosphate cleaner to wash the walls with. Allow washed walls to dry between 24-hours and 48-hours before painting.
Press patching plaster into any cracks or holes in the wall, using a plasterboard knife to get the holes and cracks filled and smooth.
Let the plaster dry overnight. Sand it smooth by hand with a piece of fine-grit sandpaper. Wipe off the sanding dust with a cloth
Apply masking tape to window and door trim, baseboard and crown mouldings, and other items on the wall that you don't want painted. Press the edges of the tape down firmly to prevent paint from seeping underneath.
Spread primer over the whole wall in a thin, smooth layer. Use a paint brush around the edges of the room and a roller for large expanses of wall. Let the primer dry overnight.
Roll on your textured paint, using a paint roller and pan, starting in one top corner of the wall and working your way out and down. Roll it on as you would standard paint, in overlapping vertical courses. Use a paintbrush to get the areas around the corners, floor and ceiling where the roller might not reach easily. Paint the entire wall.
Let the paint dry overnight. Apply a second layer in the same manner as the first layer. Let it dry for 24 hours before moving the furniture back in.
Stir textured paint thoroughly and completely before application.The textured particles generally sink to the bottom of the paint, and for proper application, they need to be fully suspended in the paint medium. When finished, you can leave the paint as-is, or paint the wall in any suitable colour that you desire.