You should always sand your cabinets before painting them if you want your project to look great and if you want the paint to last. Sanding must be completed to remove imperfections and create a smooth surface. Sanding is also necessary for developing small, rough ridges which create adhesion for the paint to stick to. It is a step that contributes to the longevity of the paint on your surface. A sandpaper with a grit between 150-220 should be adequate to remove imperfections as well as create ridges mentioned above. A great way to check and see if you have sanded enough is to hold the sanded piece at a low-level so that light reflects off of it. You'll know you've sanded enough if you see a dull reflection. Any glare (from previous finishes or paint) means you have more work to do.
