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Finishing the concrete
Release the concrete forms by carefully separating the joints with a small, flat prybar. Unscrew the blockout, but don't remove it unless you plan to install an undermount sink and need to finish the inside edges. Otherwise, wait until you've actually installed the countertop.
Ease sharp edges and corners of the slab by sanding with a random-orbit sander and 100-grit sandpaper. Wear a respirator to avoid inhaling the silica particles.
Etch the surface with a 1-ounce-per-gallon solution of muriatic acid and water. For tops made with pigmented concrete, such as this charcoal-gray unit, the acid wash exposes more of the color and texture variations. Be sure to follow the safety precautions on the acid label regarding eye and skin protection and ventilation. When you are done, rinse the countertop slab thoroughly with water to remove the acid residue, then let it air dry.
Prepare a mixture of Portland cement, latex additive and liquid pigment to the consistency of peanut butter and apply it to the exposed surfaces using a rubber-faced grout float. Then plow (or skiff) the surfaces with the leading edge of the float raised so the material remains in all of the small voids but is removed from the flats. After this filler dries and cures for an hour, sand it with an orbital sander and 180-grit discs until you are satisfied with the smoothness. Plan on going through a lot of discs!