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Remove the 2x4 support frame from the sides and ends of the mold.
Carefully drill two 2-inch screws equal distance apart, halfway into each of the melamine sides. Be careful not to drill all the way through – you don’t want to disturb the mold edge.
Remove the screws holding the mold sides to the base. Then use a hammer and the new screws to pry each side away from the concrete slab. Take your time with this step. You don’t want a misstep that will cause a chip or any breakage.
Get help from another person or two to flip the slab over. Remove the cutout sides using the same technique. The weight of the slab usually makes removing the melamine base an easier task.
When the slab comes out of the mold, it’s going to have imperfections. Use an orbital sander to remove any imperfections along the surface and edges.
Wearing a respirator, start with 100-grit sandpaper. You want a standard, smooth edge. Keep one hand on the top of the sander as you polish the edges. Be prepared; you’ll go through plenty of sandpaper and this is a dusty process. Work to progressively finer grits, finishing with 220-grit.
Sand evenly, checking the smoothness with your hand. Keep sanding and testing until each edge and surface feels smooth to the touch. When done, wipe the slab with a damp rag to remove any loose grit and concrete dust.
Prepare the surface for finishing by etching it with a solution made from 1 ounce of muriatic acid mixed in 1 gallon of water. Wipe the surface thoroughly with a sponge dipped in the acid solution. Wear eye protection, a respirator and acid-proof gloves. If you’re working inside, open doors and windows for better ventilation.
Rinse the slab with fresh water to remove the acid mixture, and let it dry completely.
Apply a concrete sealer using a sponge or brush. Work in long strokes from one edge to the other. Let the sealer dry then apply a second coat, working at right angles to the first. Keep applying coats until the concrete won’t absorb any more. Let it dry thoroughly, about thirty minutes.
Prepare the installation by running a thick bead of silicone caulk around the upper edge of the cabinet.
Bring the countertop in, set it in place, and press down gently to seal the caulk.