A hairline crack is one description of a crack that hasn't opened up. That implies there isn't much room to fill it with anything other than an extremely low-viscosity liquid. Because most hairline fractures do not compromise the structural integrity of a countertop, the main aims of a repair are to seal and conceal the crack. Cracks can be repaired using epoxy resins, acrylics, or silicone caulks. Hairline cracks can be difficult to detect because they often do not extend all the way through the slab. If you walk on the surface you will not notice them unless you use a penetrant dye which will stain the concrete where the crack exists. These stains should never be removed because they indicate a place where water may have been trapped under the slab. This could become a future problem area if the crack gets deeper or another one is created nearby. You should also never patch these stains because that will only create a new problem area. Crack sealing is done to protect any repairs made to the concrete from further deterioration. Sealing helps prevent moisture from getting into the crack which can cause the paste used for repairs to wash away. Sealing also prevents objects such as dirt and dust from getting into the crack which could affect how the repair material sets up. Silicone caulks are the most common type of material used for hairline crack repairs.