All too often, when faced with some sort of pipe thread, people want to reach for the teflon tape or paste. When you're dealing with regular pipe thread, that's fine.
But when you have a compression fitting, putting some sort of seal on the threads is the last thing you want to do. Unlike pipe threads, compression threads don't actually create a seal to stop water from leaking. The threads on a compression nut are there as a "tool" to pull a ferrule or beveled gasket towards a matching surface that creates the seal.
But, if you apply seal tape or paste to these threads, you'll actually be preventing the compression nut from fully drawing the ferrule or gasket to the other surface. You'll have more of a leak... not less.
Have you ever wondered what the difference is between these two types of drain pipe? Obviously, ABS is black and PVC-DWV is white. Both are perfect for drain systems and are both a thicker wall product, also referred to as Schedule 40. The term "Schedule 40" is often misused to mean "pressure" pipe. The thickness is all the schedule rating refers to and has nothing to do with whether a pipe is rated for pressure or not. In fact, if you look at the writing on the side of either one of these drain pipes, it will say "Sch 40 -- NOT FOR PRESSURE".
But how are they different? First of all, ABS is a derivative of oil where PVC is a derivative of natural gas. This explains why these similar products can sometimes have drastically different prices. If oil is expensive and natural gas is cheap, your better off using PVC-DWV.
They also use different kinds of glue. PVC glue requires a primer and ABS glue uses no primer. You'll also find that the pipe from one type will fit perfectly in the fitting of the other, but you need to be careful if you're going to mix the two.
First thing to remember is that mixing the two, ABS and PVC, will more than likely get your plumbing job "red tagged" if you're being inspected. The only exception currently allowed in local code is one "transition" in a line. If you choose to do that, make sure that you use green Transition Cement. If you use the white All-Purpose Cement.... RED TAG!
I'd like to blame this on New Math, but it's not New Math's fault. The fact of the matter is that there is more than one PVC drain pipe that is defined as "3 inches", and they are actually two different sizes.
The next time you're digging up a broken drain line in order to fix it, make sure you take a close look at it before slogging your muddy feet into the store to get parts. The two different types of PVC drain pipe are the thicker walled PVC-DWV and the thinner walled PVC Styrene. Both are white, both claim to be 3 inches and both have their own specific fittings that are NOT interchangeable.
It would be nice if you're able to just look down into the muddy (and likely smelly) ditch you've just dug and see the words "PVC-DWV" boldly smiling up at you, but that's not likely to happen. You could lay a tape measure down next to the pipe and eyeball the outside diameter and maybe get it right. Or, since this is a broken drain pipe, just break off a chunk of the broken pipe and bring it in with you. And if this is a sewer drain line, we'd appreciate you hosing it off first... and maybe those muddy boots, too.