Before you paint, it's important to make sure that your walls have been prepared. Older walls need sanding or old bits of flaking paint to be scraped off. Fill any cracks and gaps with filler. Let it dry and then sand it back again. When you're sanding, use a fine grade sandpaper, such as 120-grit. With your dust mask and safety glasses on, hand sand for small areas or, for larger areas, use an orbital sander.