Granite is more expensive than most other stone materials, but its beautiful look and public perception make it a sought-after choice for surfaces such as countertops. Although granite is fairly rugged and durable, it is not unheard of to have to make a repair at some point. Hairline cracks are especially common in granite that is improperly installed or when there has been a sudden and forceful impact against the surface. Hairline cracks are the easiest form of damage to repair and hide, and doing so will help you avoid a costly replacement.
Wipe the counter down with a damp sponge to remove any dust, debris or other foreign material.
Apply a few drops of acetone to a clean rag and wipe it over the scratch. Feather the acetone out from the scratch approximately 1 inch on either side. Allow the granite surface to air-dry completely.
Squeeze a thin line of cyanoacrylate into the crack. The adhesive will seep into the crack and fill it totally. Use enough of the substance so that it overfills and doesn't completely sink into the defect. The adhesive will spread slightly as it dries and flares out across the granite.
Scrape excess cyanoacrylate away with a razor blade. Hold the razor blade with the edge at a 90-degree angle to the granite surface and evenly drag it across in a sweeping motion to remove the raised area and leave the granite smooth. Be careful not to dig the razor blade into the cyanoacrylate. If you damage the crack repair with the blade, you may need to add more cyanoacrylate to the crack and repeat the process.
Colour the crack with a permanent marker to more closely match the colour of the rest of the granite. This isn't always needed for hairline cracks, as they are often more difficult to see than larger defects are once the repair is complete. Let the repair dry completely over night.
Seal the stone with a granite wax or sealer, following the product instructions. When the sealer or wax is dry, buff the entire piece of granite with a soft cloth until the surface shines and the scratch is virtually undetectable.
Apply a granite sealer every year to keep the surface looking good and also to protect the surface from further cracks.
Sealer alone won't repair or hide the crack, as it easily soaks into the porous unsealed stone rather than filling the crack.