Easing the edges around sink openings in concrete countertops is important to prevent chipping and to provide clients with a smooth, comfortably rounded edge. There are a couple of ways to do this: shape the edge by hand after casting, or form the roundover before casting.
While shaping the cast concrete is practical for straight edges or outside curves, it’s often difficult to do on tight, inside curves like those found in an undermount sink opening. And getting a smooth, even curve does take some practice, even with a special grinder and roundover bit.
One way to form a roundover before casting is to run a bead of silicone caulk and carefully smooth it to the right radius, and then let the caulk cure, usually overnight. For details on this method, see this video.
Here is another option: Get some non-hardening plastic modeling clay from the craft store. Any color will do! Roll out a rope of clay about 1/4″ diameter (or more for bigger roundovers) and pack it into the corner or edge to be rounded over.
Next, make a shaped scraper. A piece of thin wood template stock works, but an even better (and free) scraper is made from a laminate sample chip. Sand or file a different radius onto each of the 4 corners: 1/8″, 3/16″, 1/4″, 3/8″, or whatever you want. You can even do bevels or other shapes too.
Once the clay is packed in the corner between the form side and the casting table, scrape the excess clay away using the scraper. The radiussed corner will cut and scrape a smooth, even shape into the clay. The form is then immediately ready for oiling and casting.
Once stripped, the clay can be pulled from the concrete and reused. A degreaser like Greased Lightning makes short work of any clay residue, and a power washer gets out any clay that becomes smeared into the voids.
A big disadvantage of this method is that modeling clay is oil based, and it will discolor the edge of the concrete. Silicone caulk will not cause any discoloration. If you are grinding to expose aggregate, or doing heavy honing to expose sand grains, the discoloration will likely disappear. Do not use modeling clay if you are leaving a cream finish on the concrete.