The old J pipe basically disintegrated in my hand when I pressed my thumb against the leak. So I bought a new one, matching it exactly to the metal one I removed. I asked them at Lowe's about joint compound and they said use plumber's putty. Here is my question: How does one use plumber's putty? This may sound silly but I have never done any plumbing repairs and while the J pipe fits snugly, I don't want it to leak again. Please instruct me and thank you for your time.

This is why big box salesmen aren't out there doing plumbing. YOU DON'T USE PUTTY on a compression joint. You use the washers that came with the "J" bend. Place the compression nut and washer on the tail piece coming out of the sink and the other washer on the piece coming out of the wall. Make sure that the washer goes UNDER the lip. Start the nuts being careful not to cross thread. Now take a pair of large pliers and tighten the nuts until they are snug. Stop off a tub and run a few inches into it. Let the water drain out and check the trap for leaks. If you see any leak arounds the nut snug them up a little more. Good luck, Tom

The Home Depot guy said "We're not a hard ware store".

Me, I go the extra mile and use some silicone grease (In plumber's Asile) on the threads and the compression washer. Aids in removal and helps the seal.

Drain cleaner and just plain time loves to eat through these metal pipes.

You DO NOT use putty, grease, or silicon jel on compression nut threads. Since the threads don't hold anything but the compression washer it's useless to load them up with anything and if the joints line up on the compression fittings you don't need to add anything there either. Sorry, Keep it simple. Tom

Tom:

Good vacuum practice and automobile practice and refrigeration practice says to grease the "O" rings with a compatible fluid/grease. Why does plumbing have to be different? I know the rubber washer is flat, but the principle applies. As you tighten a non lubercated fitting, there is a tendency for the washer to get distorted. Grease prevents that process.

The silicon grease on the threads assures easy removal at a later time.

Not required. Just helpful
Try this test:Take a trap and grease the threads. Take another trap and don't grease the threads. Wait 20 years. Which trap will be easier to remove?

Silicon grease does no harm. In fact, it makes the next plumber's job potentially easier.

Note here prevents the rubber from drying out:

Silicone Grease and Compounds (http://zero-friction.com/siliconegrease.aspx)

and this one:

Select Products Plumbers Grease (http://www.selectselect.net/acatalog/Plumbers_Grease.html)

which states:

Plumbers Grease Lubricates & Protects metal, rubber & Plastic parts. (Use on "O" rings & value stems)

Note the word metal which the P trap is made of.

Sorry Tom, I still disagree. Hopefully I've provided proof that it's a "GOOD IDEA".

If easy removal is a good thing.
If lubercation makes easy removal.
If silicon grease is a good lubercant.
If silicon grease is compatible with water systems.
If silicon grease prevents rubber/"O" rings/gaskets from drying out.
If a metal P trap contains a rubber gasket.
If a metal P trap is made of metal

Then silicon grease can be used on metal "P" traps to prevent gaskets from drying out and also the facilitate removal of the fittings at a later date.

KeepitSimple,
You make a valid point. Although I've never used used lubricants on compression fittings and still advise against putting any kind of lubricant on plastic threads you have a point well taken. Thank you for correcting me. Tom

I used Keepitsimplestupid's advice and it put an end to many hours of hassling over my kitchen sink problem. The rubber tail piece gasket kept twisting as I tightened the nut and a little grease is all I needed to end the problem. I tried silicon caulk on the outside, but that didn't work and the more you tighten the worst the leak got. A little bit of grease worked wonders. Thanks Keepitsimplestupid!! The whole internet and no one else mentioned this... That is plain stupid.