Porcelain is made from metal and glass that is fused together with heat. It is a popular choice for bathtubs and sinks; it's durable and beautiful, but can chip or crack if it is damaged. You can repair a crack in a porcelain sink yourself if it is in an area on the sink that stays dry. However, in areas that stay wet, like inside the basin, the repair will likely not hold.
Make sure the sink is clean and dry before beginning your repair.
Sand the area in the sink that needs repair with the sandpaper until the area is smooth.
Dampen a cloth with rubbing alcohol and wipe the area you sanded to remove grit and debris. Let the sink dry completely.
Mix a small amount of the paint and porcelain repair compound on a piece of tile using the tongue depressor. Follow the directions on the repair compound for amounts to use.
Scoop up a small amount of the compound and paint with the razor blade and apply it to the crack in the sink, filling it in completely and just overlapping the edges of the crack.
Dip a cotton swap in the fingernail polish remover and use it to blend the edges of the paint and compound into the porcelain. Let the repaired area dry completely before using the sink.