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You’ll use the float glass as the “main” work surface on which you’ll grind your glass edges. The foam rubber will serve to keep the float glass in place so it doesn’t slide off during the grinding process. If you don’t have a dedicated piece of float glass, you can also use a piece of scrap plate glass, such as a piece of window, mirror, or picture glass. Pour a small amount of water into the center of your float glass to create a small puddle. Then, pour in just enough silicon carbide to cover the surface of the puddle. Finally, use your fingers to gently stir the silicon carbide and water together. To make it easier to add the silicon carbide powder, place it in a small disposable cup first before you begin adding it to the glass. Grasp the piece of glass with your dominant hand (or with both hands, if needed). Be sure to place the glass edge directly over the spot where you mixed the silicon carbide with the water. For maximum safety, wear safety gloves to avoid cutting your hands on the glass. Move the glass over and through the silicon carbide in a figure eight motion, applying gentle downward pressure as you do so. Make sure to stay within the glass surface and avoid going beyond the silicon carbide puddle. Look at the edge of the glass after about a minute of this circling. If the glass is no longer glossy and it is smooth to the touch all around, you are done. Use a wet paper towel or a dedicated cloth towel to remove the silicon carbide water from the glass piece. Then, use wet sandpaper as necessary to smooth out the inner corner of the edge of the glass if needed.  For example, if you’re smoothing the bottom of a cut glass bottle, you won’t be able to smooth the inner corners of the glass edge with the silicon carbide puddle. You can also use sandpaper to sand down any rough spots you find that the silicon carbide didn’t get.

Summary:
Place a piece of float glass over foam rubber on your work surface. Add water and a coarse grit of silicon carbide powder to the glass. Place the sharp edge of the glass against the plate glass. Circle your hand and the bottle around the glass for 30-60 seconds. Wipe off the glass with a towel and use sandpaper to smooth the inside.