having worked around piping of one sort or another in powerplants and on locomotives and other jobs, here is what thirty odd years of experience has taught me:
1. A pipe thread is a tapered sharp vee thread. It is intended to be a "dry seal thread". That is, as it is made up, the taper and the sharp vee thread forms combine to make the threads self-sealing- IN THEORY.
2. If pipe threads are properly cut and not torn, burred, and are "virgins"- having never been together and apart- then the only thing the pipe dope (pipe joint compound) is really supposed to be doing is to act to lubricate the threads and to prevent galling or seizing of the threads when they are made up. The threads, being cut on a taper, will wedge tighter as they are made up. This produces a hard rubbing along the flanks fo the threads. Aside from the friction which is going to occur, there may be a galling or seizing if the male & female threads are similar materials. Older pipe dopes were nothing more than linseed oil + white lead, or linseed oil + litharge, or steam cylinder oil + graphite.
3. Once pipe threads have been made up, the male threads may tend to spread the female threads a bit. These take a "set". Break the joint apart and reassemble and it may leak. That is when the pipe joint compound ( pipe dope) gets into more of a sealing role.
That is also where Teflon Tape comes into the act. Teflon tape is a thin film which is supposed to act to lubricate the threads and not migrat einto the fluid int he piping. For potable water, this is ideal- who wants to be putting linseed oil, Permatex, or similar into the drinking water ?
The downsides to Teflon Tape are numerous and it is simply not allowed on some jobs. Teflon tape is one of the most misunderstood and misused things out there. People tend to wrap it on heavy on the male pipe threads. The result is it shims and wedge/spreads the feamle fitting threads. People tend to wrap that old Teflont ape on and leave some sticking over the lead threads. Get things together and the Teflon tape gets cut by the leading thread. Shreds of Teflon tape wind up in the fluido or gas flow. The result is Teflon tape inside of control valves or insturment needle vavles or worse.
Get the Teflon tape wrapped on heavy with a cross or two in it on the male threads, make thing sup, and the Teflon tape will tend to extrude and form unevenly on the flanks of the threads. On brass or steel fittings this is not a problem. On PVC threads, it is often the end of any hope of good sealing.
Teflon tape has its place, but it must be carefully used.
4. PVC pipe is a whole 'nother story: it is softer and deforms easily. Making up PVC screwed threads to metal fittings gets interesting as the metal threads will tend to deform the PVC if made up too tightly. Add some Teflon tape and a shim on the threads has been added. Once the PVC fittings and pipe are made up with Teflon tape, a permanent deformation of the threads and female tapers may occur. Leakage will follow.
What this leads to is my attempt to answer Gauge's question as to how one ought to be making up PVC screwed pipe threads and fittings. My own recommendation would be to get a PASTE-TYPE Teflon pipe joint compound for potable water service. If the PVC threads screw onto or into steel, glavnized steel, iron, brass or copper fittings, then use the paste type pipe dope. Apply it to the male threads. Make sure the threads are all cleaned out to remove any shreds of the Teflon tape and any burrs or other debris.
Screw the PVC and metal pipe threads together by hand. If over about 1" pipe, use a small pipe wrench and just snug things up. You do not need to put a whole lot of torque onto things to have the tapers start wedging and deforming. After things are mad eup, try putting a compressed air test onto the piping if possible. Check for leaks with soapy water. Air will pass thru a tiny leak that water will not. It is easier to fix piping that has had air in it than water.
OK- assuming you do have a few tiny little pisser type leaks. You can try taking up on the joints (tightening) a little bit, but you must be careful not to start some other joint to leaking. If you used the pasty type of pipe dope, let the piping sit pressurized for an overnite or two. Often, the paste type dope will work out and stop the leak of and on its own.
Now for the oldtimer's fix. This dates to the days of black iron fittings and steam radiators. It is not according to Hoyle, but may enable you to save the fittings and piping you have. If the fittings have been mauled or spread due to overtightening and Teflon tape, this is a fix which may let a person get leaks dired up. It worked on old steam radiator piping which had been made up using big pipe wrenches, joints made up and taken apart a few times to get the condensate out of heating radiators... joints made up by guys with the idea that the only way to make up piping was to bend or bust a pipe wrench. This spread the fittings and then some. The piping used to leak. The old plumbers had a remedy and the word for it is "Wicking" (or as the old plumbers would say "Wickin'"- not to be confused with the belief in witches). Go into an independent plumbing supply house and find an older counterman. Ask him for a ball of fine wicking or "candle wicking". It is a kind of soft, loosely spun cotton thread. Get some teflon based paste type pipe joint compound (pipe dope). Now you can do what the old plumbers did and probably get away with it. Take your piping apart and clean everything real good. Paint a thin coat of the pipe dope on the first male thread to go together. Wipe the dope into the threads with your finger. Take that ball of wickin and carefully wind it onto the threads so it lays neatly in the "valleys" of the threads. It must not cross any threads. Pull the wickin so it lays in nice and tight with no slop. Make an extra turn or two around the last thread to hold the wickin in place from unravelling. Apply another thin coating of the pipe dope over the wickin. When this is done, make up the threads, bringing things snug, then tighten up by HAND if running PVC threads on metal threads. If over 1" pipe threads, use a small pipe wrench.
Keep going as you assemble the piping. Remember to use a wrench as a "hold back" so that you do not distrub the joints you already made up as you work on joints further downt he line.
I learned about making up "used" Dad's Rigid pipe wrenches, Rigid and Reed pipe cutters, die stocks and a few balls of wickin'. We lived in a four family house, so I helped my dad do plenty of plumbing repairs in the tennant apartments and we would get called to help out the neighbors up and down the block. All screwed red brass, or black piping in those days. Still, if it was together and apart, especially if dad saw the wrench marks from some previous do it yourself/gorilla who had tried to "fix da leak", dad knew it had a good chance of leaking. Dad would tell me to wind in the wicking and dope 'er up. In those days we used "Hercules" pipe dope- one type for all uses. When we got done, the joints held good.
When all else fails on PVC/copper or PVC/Brass screwed joints on my well system, I go get the ball of wickin. I learned about the wickin' when I was a kid, so this is going back 40+ years. On powerhouse work and locomotive work, the wicking is not used. Wicking is for residential plumbing.
Another oldtime pipefitter's and plumber's tip: Always put a nipple into the un-used female threads in the branch of a tee when making it up. Similarly, put a nipple into the far end of any valves as you make them up. If you don;t and get to tightening, you could deform the tee or valve body. Old steamfitters showed me this years ago and it made sense to me.
The main thing with any of this work is to take care and make the joints up without geting too rammy. PVC pipe threads simply will not hold up when wedged hard against metal pipe threads. What seemed a normal amount of torque for tightening a brass or galvanized ipe fitting on a similar nipple or piece of pipe will deform the PVC threads. Take the whole run of piping apart if you are havng leaks, inspect it for damaged threads and try remaking the joints using a paste type dope plus wicking. If you work your way along the piping and use a "holdback" wrench to avoid putting any more tightening (or loosening) torque into the joints you already made, that will also help a lot.