When you drop something onto a porcelain tile floor or hit a porcelain tile wall, the tile should generally chip instead of cracking, according to Ceramic Tile and Stone Consultants. If the tile cracks, it was likely installed directly over a concrete floor and has spots beneath it that lack adhesive. While it can be difficult to correct the issue of missing adhesive under a porcelain tile floor, you can repair hairline cracks from the top of the tile.
Fill the crack with clear epoxy or caulk. Use a putty knife to spread the epoxy or caulk over the tile, and scrape the side of the knife across the tile to remove the extra epoxy or caulk.
Refill the crack again if the epoxy or caulk sinks down into the crack. You may need to fill the crack a few times before it dries level with the rest of the tile's surface.
Run your fingers across the surface of the tile. If the surface feels uneven where the epoxy or caulk was added, sand the surface very gently with fine grit sandpaper. Try to sand only where the epoxy or caulk is, avoiding the tile's surface as much as possible to prevent scratching.
Paint over the area with the epoxy or caulk if the filled crack is still visible. Dab just enough waterproof paint onto the surface of the epoxy or caulk to make it blend in with the rest of the tile's surface.
Porcelain repair kits are available at home improvement centres as well. Repair kits contain a resin base with acrylic paint to match most common colours. Apply according to the directions on the package.