Moisten a scrub brush with water. Sprinkle powdered household detergent over the scrub brush bristles.
Scrub the nicotine stains on your porcelain sink. Rinse the scrub brush frequently. Apply more powdered detergent, and continue to scrub to remove as much of the stain as possible.
Rinse the sink with water to wash away the detergent and loosened nicotine.
Make a poultice to tackle any remaining nicotine stains. Pour 1/4 cup of whiting and 1/4 cup of household ammonia into a bowl. Stir the mixture with a spoon.
Apply a spoonful of the paste to the remaining stains on your porcelain sink. Cover the stains entirely. Allow the poultice to remain on the porcelain for one hour.
Wet a nylon mesh sponge with water. Scrub the poultice-covered area. Rinse the porcelain with water, and dry the sink with a clean cloth.