I have a bathroom sink that leaks. Any suggestions on how best to seal this leak? The water leaks only from the bottom of the bowl when I run the water. The sink itself is about 70 years old - porcelain. I used Plumbers Putty and left the sink alone for 24 hours. The putty but it peeled off. Any suggestions? Many Thanks!

Is the water leaking from where the drain goes through the sink or somewhere else ?

Sounds like the putty seal under the drain lip is shot.
I've put up a image of the pop-up assembly less the lavatory bowl. Purchase a small can of plumbers putty.
Now loosen the nut that secures the drain enough to liftthe drain up a bit. Clean out the old putty and run a ring of new putty about a quarter inch or so under th lip. Tighten the nut until the extra putty squeezes out. Snug the nut up some more and you're done. Leak all better now? Tom

Yes. The drip is right under the drain on the underside of the sink.

Thanks speedball1 for your help. Unfortunately, there is no nut under my sink. Must have fallen off years ago. I only have what looks like two very worn out washers. Any other suggestions?

yes. the drip is right under the drain on the underside of the sink.

thanks speedball1 for your help. unfortunately, there is no nut under my sink. must have fallen off years ago. i only have what looks like two very worn out washers. any other suggestions?
Yep! Install a new pop-up. I can walk you through it if you wish. Cheers, Tom