A most common causes of a leaking bathroom sink drain are the rubber seal located beneath the drain crumpling due to age, and the seal beneath the flange of the drain breaking down. To seal the bathroom sink drain, you will need to remove the sink drain and replace the bottom rubber seal and the reseal the drain flange. This is the only way to ensure that the bathroom sink drain is completely sealed.
Loosen the locknut that secures the bathroom sink drain to the bottom of the sink bowl with the pipe wrench.
Turn the locknut counter-clockwise to remove the locknut from the threaded section of the drain assembly.
Push toward the sink with one hand to free the drain from the sink.
Pull the loose sink drain from inside of the sink and place it flange down on top of the bathroom vanity.
Scrape the bottom of the sink drain flange and the inside recessed area of the sink bowl with the two-inch putty knife to remove the old sealant from both areas.
Rub a quarter-sized ball of plumber's putty between your hands to form the putty into a rope around eight inches long.
Place the plumber's putty rope in the recess of the bathroom sink, and form the rope into a circle that surrounds the opening in the bottom of the recess.
Slide the drain into the bathroom sink opening and press down on the drain flange to seat it into position.
Crawl under the bathroom sink and slide the new rubber washer over the threaded section of the sink drain.
Thread the locknut onto the threaded section of the drain and tighten it down with the pipe wrench.
Slide the two-inch putty knife around the area where the sink drain flange and the sink meet to remove the plumber's putty that has oozed from under the sink drain flange while tightening the locknut.
Turn on the water and check underneath the sink for any water leaks. If you notice any water leaks, tighten the locknut with the pipe wrench to better seat the bathroom sink drain.