While tile is a sleek, durable material for kitchen floors and bathroom walls, it's not indestructible and can chip or crack over time. It can also crack if the material underneath the tile is unstable and begins to shift slowly over time. While most experts believe that the best way to repair any size crack is by replacing the tile completely, there is at least one method to fix a small hairline crack. However, keep in mind that any crack in your tile is often a sign of a larger problem---namely, an unstable foundation.
Squeeze the two-part epoxy out into a plastic plate or bowl and mix each component together with a wooden stick until they are well-blended.
Dip your brush into the epoxy, dabbing the brush directly onto the hairline crack. Wait for the adhesive to dry and then wipe away any excess epoxy with a rag.
Assess whether the tile could use a coat of paint so that it matches the other tiles better. Some hairline cracks are so small that paint isn't necessary.
If so, purchase paint that is suitable for tile. Try to find a shade that matches your original tile as closely as possible. While you're there at the hardware store, purchase some clear coat sealer for after the paint dries.
Brush some paint very lightly over the crack so that it's layered, yet not too thick. Allow the paint to dry according to the directions on the can or package and then dab on a layer of one-coat sealer to protect the paint once it has dried.