Sandpaper comes in a range of grades. The grade is indicated by a number with rougher sandpaper having a lower number than fine sandpaper. The roughness of the sandpaper comes from the coarseness of the grit stuck to the paper. To get a perfect finish on your walls, you will need to use two grades of sandpaper.
If you are sanding a patch of plaster used to repair a large hole, then you should first make sure the plaster is fully dry. The surface may have dimples or cracks after drying. Fill these in with filler before sanding. Apply the filler with a spatula and scrape off any excess. Don’t worry about getting the surface completely flat with the spatula; the sanding process will achieve that. Let the filler dry hard before sanding.
You will get a smoother finish if you use a sander. Sanding by hand can be a laborious process and tiredness can lead to sloppiness. Sanding with a machine is faster and so fatigue will not slip in. You can buy sandpaper strips specifically designed for your sander. However, if it fixes the paper at each end with a clamp, you can buy regular sheets and cut them to the size of the sander’s pad.
You should pass over the walls first. The first pass will sand off any lumps and bumps with a rough sandpaper of grade 120 or 150. This rougher sandpaper will leave scratches. Next, you will have to pass over the wall with a finer grade to smooth the scratches left by the rougher sandpaper. A 220 grade will smooth the walls nicely. If the walls are in very bad condition, you may need to make three passes, using 80 grade paper on the first pass before passing over the walls with the 120 or 150 grade paper.
Sanding walls produces a lot of fine dust. Be aware that the room will fill with this dust even if your sander has a dust bag. Open all windows and doors in the room to invite in cross drafts to clear the air. It is advisable to remove all furniture, carpets and curtains from the room. Even if you will be sanding at a distance from the furniture in the room, the dust will drift and give you extra work you clean the furniture if you don’t remove it. Wear goggles and a disposable mask to keep the dust out of your eyes, nose and mouth.