If you use this flux, The only way you will have a leak is if you forget to solder the damn thing, it pre cleans and tins . The Nick solder is fantastic. It is lead free but flows like it is lead solder
i love the stuff and these are what you find on my truck
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https://www.ferguson.com/product/jw-harris-nick-nickel-solder-hnick61/_/A-ProdFamily-221103
use C-flux and Nick solder
The tricks to copper piping is CLEAN clean the pipe and inside of the fitting
do not touch the pipe ends after cleaning, the oil on your fingers will cause the flux to not take at that spot
after soldering you MUST wipe the joints wih a damp rag to remove unused flux
You question on soldering in tight spots. you actually answered you own question.
the solder wicks around the pipe.
the main trick is your torch. those big ass flame throwers they sell at the big box stores do not allow you control over your heat. it is off or on..
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Above is a pos flame thrower, Below This is the second choice
https://www.plumbingsupplynow.com/stk-9-torch-swirl-for-use-with-map-pro-or-lp-gas-0386-0403?msclkid=055a0d9636741781af6053a852ef7faa&utm_source=bing&utm_medium=cpc&utm_campaign=**LP Bing Shopping All Products&utm_term=4574861726366157&utm_content=Catch All
First choice, The one that will do every thing you want will run you about 200 bucks
https://www.plumbersstock.com/uniweld-k37-acetylene-b-tank-brazing-kit-wth3-sc.html?
utm_source=bing&utm_medium=cpc&scid=scbplp6665&sc_intid=6665
that is the torch, then you need a ''B" tank
https://www.riogrande.com/product/BAcetyleneTank/500060?msclkid=31583b3143c51f1d8b3d650f25cd4ca8&utm_source=bing&utm_medium=cpc&utm_campaign=Shopping_Tools & Equipment&utm_term=4584963490469890&utm_content=Tools & Equipment
I have a tank I will sell for 50 bucks. but shipping would kill that deal
the better the torch the better the solder job.
heat the back side bottom of fitting and solder the opposite side top
the solder will run to the heat.
I tippy tap. While i am heating i am tapping the pipe with my solder. when it starts to run I pull the heat away
when soldering a vertical joint, solder the top first then the bottom
TIP
if you have to solder in a tight spot up against something
heat the pipe 2 or 3 inches back. it will heat the joint. takes a minute longer . no biggie