White porcelain sinks that are chipped or stained can be repaired. For a dark surface stain on a sink or counter top, try sanding with the finest grain sandpaper you can find. Be careful not to sand through to the dark metal undersurface of a sink.
Repair kits for chips are available at home centers and come in white and assorted coklor. They are not recommended for areas that are continually exposed to water, such as the area around the drain or the faucet base.
