Brass pipe + WICKING

What is it about BRASS pipe that requires adding "wicking" or string to the threads to get it to SEAL properly.
I had to install a TP valve on a hot water heater for a friend of mine and used string along with the GRAY pipe dope of old.
Often on bigger fittings brass or not , 3 or 4 wraps of Teflon tape doesn't do the job and LEAKS. I had that problem on my furnace boiler at the pump connection. Add more teflon tape, still leaks. I took it off again and used gray dope - no more leaks for 20 years.
I assemble galvanized and black pipe with teflon tape and no problems.
It seems only BRASS requires "wicking" and dope together.
WHY since threads are the same ?
TIA
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Been a steam and pipe fitter for 40+ years (now retired) and the only time I encountered string on threaded fitting was in sloppy DIY work.. Poorly cut threads or out of round fittings. Seldom did the job and were still leaking when I took over the job..
First of all pipe threads are tapered and the only reason you use pipe dope or teflon is to lubricate the threads as they tighten up.. The more you tighten a tappered thread the tighter the fit and the more friction.. You need a lubricat that can with stand the ever increasing pressure.
Brass threads often gall as they are tightened and you really need the thread dope or teflon..
Check your threads, both male and female to verify that they are good and that both are round..
--
My opinion and experience. FWIW

Steve



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Conase wrote:

the reason you had to use anything on the brass threads is because it was used once before... you can take an old brass fitting and put it on a fitting or pipe and do it by hand, but go get a new brass fitting and try that, the only thing you gonna do is get the firs thread or two on by hand and need the wrench for the rest.... the old brass fitting is distored from using it.... remember that brass is soft metal and will follow the shape of the steel pipe you are putting it onto.....
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