The next stage is to make sure that the area under the wall is covered, as the job itself is a messy one. You can use dustsheets for this, which will enable the cleaning up process at the end to be a lot quicker and easier. You will need to start with 60-grit sandpaper, which is available from most hardware stores. This sandpaper is a medium sandpaper, 50-grit is too coarse for the job and will actually damage the wall. Attach the sandpaper to the sander and sand the entire wall, making sure to apply even pressure throughout, the easiest way to achieve this to move the sander in a circular motion.
Wipe the wall down again to remove any new dirt from the first sanding. Replace the 60-grit sandpaper with 100-grit sandpaper and repeat the process, again ensuring that even pressure is applied. 100-grit sandpaper is fine and is normally used for sanding bare wood, although it is suitable to use in this process, as the 60-grit sandpaper will have removed the majority of the texture. Wipe the wall clean again.
Next, you will need to replace the 100-grit sandpaper with 150-grit 180-grit sandpaper and repeat the sanding. The 150-grit and 180-grit sandpaper is very fine and will smooth out the remaining bumps on the wall. Wipe the wall clean again and your wall should be ready for the fresh paint. Replace the dustsheets and paint the smooth wall.