Sandpaper is measured according to grit, with 3,000-grit being the finest. As the numbers get lower, the sandpaper becomes coarser. If you want to know which grit sandpaper to use to remove swirls, it depends on which material you want to remove the swirls from. When you're working on a car, for example, you want to use extremely fine sandpaper grit, but when you're working on a floor you can use a coarser grit, such as 16-, 36- or 50-grit. If you sand a floor through a three-step process, you can start with 20-grit, sand it again with 80-grit and then go over it once more with 100-grit. Depending on which stage of a paint job on a car you're working on, you'll want to choose different types of sandpaper; if you're sanding the primer, you might use 400- or 600-grit sandpaper, and if you're sanding after painting, 1500-grit sandpaper is a good choice.