The pliable, oily putty that plumbers use to seal the underside of the strainer when they install a sink drain doesn't have many other plumbing uses. Plumber's putty, as this substance is called, forms a watertight seal, but it takes a long time to harden. Its flexibility makes it ideal for sealing sink drains but not much use for patching holes in pipes, carrying water under pressure.
Description of Plumber's Putty
Available in pint and half-pint containers at any hardware store, plumber's putty contains clay or limestone as a base ingredient. It is usually mixed with linseed oil to help it maintain its pliability and prevent it from drying out. The oil also repels water, giving the putty the ability to form a watertight seal under the right conditions. Primary among those conditions is the compression formed at the joint of two surfaces, such as a sink and the underside of a sink strainer held tightly in place by a nut. There is no glue in plumber's putty, and it doesn't adhere well on its own.
Patching a Pipe with Plumber's Putty
Attempting to patch a leaky pipe with plumber's putty is likely to be an exercise in futility. If water is coming out of the pipe, the putty won't stick to it at all. If you try turning off the water and letting the leak dry out before you apply it, it may stick, but it may take weeks to harden. Even if you give it this much time before turning on the water, it still won't form a seal. It has no glue, so water will seep out from under it, and it will eventually separate from the pipe.
The Right Way to Patch a Leaky Pipe
There are some sealers on the market that you can use to patch a leaky pipe, including epoxy putty and fibreglass resin tape, but they can't withstand the force of water under pressure and should be considered temporary fixes. Whether your pipes are plastic, copper or galvanised steel, the best way to fix a leak is to cut the leaking section out of the pipe and attach a new length of pipe with the appropriate couplings. If a fitting is leaking, remove it and install a new one. That way you'll be sure the leak won't reappear.
You can make an emergency repair to a leaking pipe with a piece of rubber and some pipe clamps; plumber's putty may come in handy to help make the repair effective. The repair involves wrapping the rubber around the leak and securing the ends of the rubber to the pipe with pipe clamps. If you spread plumber's putty around the pipe before you wrap the rubber, the putty may enhance the seal. Like an epoxy putty or fibreglass tape repair, though, you should consider this fix temporary and replace the pipes as soon as possible.