Whether you are sanding your countertops for the first time or refinishing old concrete countertops, the sanding methods will be the same. Sanding the countertops with an orbital sander will prevent sanding ridges or ruts. The circular motion will cause the countertop to be smooth after the final sanding. However, for larger countertops or for a quicker sanding, using a four-inch right angle grinder will suffice. This tool turns faster that does an orbital sander and will hold up to harsher conditions, such as rough concrete or several layers of finish. Whatever the sanding tools used, you should remember to use a dust mask and eye goggles in order to prevent concrete dust from entering your eyes and lungs.
Several grades of sandpaper should be used on your concrete countertops: 60-grit paper is for sanding rough surfaces, 80-grit is for evening the sand marks made by 60-grit paper, and 120- and 220-grit papers bring the countertops to a smooth finish. These papers can be purchased to fit on orbital sanders and right angle grinders. Try to use papers that are reinforced with cloth fibers so that you can sand wet areas. Wetting the concrete countertop as you sand will help keep down the concrete dust. Water will also help you to obtain a slicker finished surface.
When sanding your countertops, begin with 60-grit paper in order to remove old finish or to even out lumpy or rigid texture. Be careful with the 60-grit paper, though, because it will remove large amounts of material in a hurry. Once the countertop surface is evened out, use the 80-grit paper to refine the surface. Finally, use the 120- and 220-grit papers to produce a glass-like finish on the concrete countertop. Remember that you will need to apply a sealer to the concrete to prevent stains from penetrating the surface.