Granite slab countertops are frequently not long enough or large enough to cover an entire counter on their own. You may have to line up several pieces of stone together with an epoxy resin color-matched to the counter to fill the seams between them. After time, this epoxy can dry out, chip away and come out of the seam like old grout. To repair it, treat it like grout. Remove the old epoxy and patch it with a granite repair kit that matches the color of the granite counter.
Remove what remains of the epoxy from the original seam. Use a utility knife with a fresh blade and begin at the back of the counter, nearest the wall. Depress the blade into the seam and tug it toward you in small cuts. When the blade stops cutting easily through the epoxy, replace it with a new one.
Mix up a small amount of granite repair epoxy to a color that closely matches the granite near the seam. Place masking tape on either side of the seam, right up to the edge.
Use the applicator enclosed in the granite repair kit to smooth fresh epoxy into the seam. Apply the epoxy from both sides, dragging the stick into and over the seam. Work from the back of the counter to the front to avoid leaning on a freshly mended seam.
Peel away the masking tape and wait 10 minutes for the epoxy to begin to set. Use a razor blade to trim and clean up the seam so it lies flush with the counter. Leave the seam to dry completely overnight.