Things You'll Need
Single edge razor blade
Power sander with fine-grit sandpaper
Wooden stir stick
An ill-formed joint between slabs in a granite countertop can completely ruin the countertop's appearance, marring the clean, smooth surface of the granite. Repairing an ugly joint line is possible, though. All that's necessary is the removal of the surface epoxy making up the seam and its replacement, making sure that the epoxy is in the joint only so you end up with a smooth, continuous surface with the seam barely noticeable against the slabs.
Remove as much excess epoxy along the seam edge as possible with a single edge straight razor blade. Hold the blade against the granite joint at a 45-degree angle. Drag the blade along the seam, cutting away the epoxy placed onto and to the sides of the joint line. Wipe the surface with a wet cloth to remove residue.
Attach fine-grit sandpaper to a power sander. Sand along the seam line with the sander to remove any leftover epoxy and to make certain the two slabs meeting at the joint are level with one another. Wipe the surface again with the wet cloth to remove residue and then with a dry cloth to dry the granite.
Remove some of the epoxy placed into the joint with the edge of a putty knife. Dig into the joint with the knife edge, scraping out the top 1/8- to 1/4-inch layer of epoxy from the joint. Wipe the surface clean after removal.
Mix a small batch of 2-part epoxy adhesive, tinting the adhesive the same color as the granite countertop. Use a small plastic cup and a wooden stir stick to mix the two parts of the adhesive together in the ratio suggested by the epoxy manufacturer. Set the mix aside for about five minutes.
Place strips of masking tape along the joint edges, one on each slab making up the seam, so that only the joint is accessible.
Stir the epoxy again for about 15 seconds. Fill the joint with the epoxy adhesive, using the putty knife to press the epoxy in place. Fill the joint until it's slightly overflowing and then level the seam out by scraping over it with the edge of the putty knife. Allow the epoxy to dry for an hour and then remove the masking tape, leaving behind a neat, straight seam with no epoxy on either slab surface.