Call in an experienced plumber and ask him to install a new drain fitting.

This requires very careful cleaning and preparation beforehand, especially around the drain fittings.

Answering the Mail Q. My sink is badly stained by rust that forms around the metal rim of the drain fitting.

Relieving the pressure by removing and resetting the drain fitting should keep it from getting worse, but unfortunately I know of no way to correct or repair the existing cracks.

The most desirable antique sinks have original drain fittings, but hardware has often been replaced or updated with a mixer that allows hot and cold water to blend.

Cracked or worn washers will allow water from the drain to leak past the retaining nut on the drain fitting.

Your drain fitting may not have been clean enough, or you may not have got all the old finish off.