Kasher the sink:
• First, clean the sink and do not use it for 24 hours.
• It is preferable to kasher the sink by pouring boiling water (iruy) in conjunction with an even melubenet (a heated stone). To kasher with an even melubenet: Attach a stone to a clamp, and heat the stone on the stove for a minute. While pouring boiling water over all surfaces of the sink, move the stone along the sink with the boiling water so that the water is reheated on the surface of the sink. The stone may need to be reheated on the stovetop several times, since the stone may cool down quickly.
• In lieu of kashering with a heated stone, some will just kasher their sinks by pouring boiling water and then using a tub or sink insert.