A good, solid coat of enamel will last a lifetime with gentle use. But accidents happen and enamel will inevitably get chipped or broken from time to time. If a chip has flaked off of your sink or toilet and has left an unsightly black mark in its place, you can easily repair it yourself with a steady hand and some specialised materials.
Save the enamel chip if you can find it. Having the chip on hand will make it easier to match the proper colour paint.
Buy enamel paint that matches your sink or toilet. The easiest way to determine the exact colour is often to look on the manufacturer's website or to contact the manufacturer by phone. If a hardware or paint store near you has the technology to mix enamel paint in custom colours, you may also take the chip to the store to get a custom batch made. If you have the chip, another option is to match it to cans or swatches of paint in the store.
Shake the paint well. Use a small, fine tip paintbrush to apply a thin layer of enamel paint to the chipped area. Allow the paint to dry completely.
Fold a piece of fine-grit sandpaper into a triangle shape so that it has a fine point. Place your finger over this point and apply light pressure as you use it to sand any spots where the enamel paint has dried on the existing enamel, creating an uneven surface.
Add additional coats of enamel paint to the chipped area as necessary, but allow at least four hours of drying time between each coat. Be sure to sand as needed between coats, as well. Many chips will require at least five or six separate applications of paint before the chipped area is even with the existing enamel, so the entirety of this job may take several days.
Make sure the area that needs treatment is completely dry before painting, and make sure no moisture touches the area before the most recent coat has had at least four hours to dry.