While sanding is not necessary for every painting project, rough patches on walls should be sanded before painting in order to ensure that the paint applies smoothly, regardless of whether the wall has been painted before or not. Any uneven surfaces can be smoothed out with sandpaper or a sanding block. Then, use a moist cloth to remove any dust.
For surfaces that have previously been painted with gloss or high-gloss paint, smoothing the walls before painting is required. Use fine-grit sandpaper to remove any water-based paint that has already been painted. A medium-grit sandpaper (100 to 150-grit) should be used for oil-based paint. Once more, wipe away any dirt and give it time to dry completely.