What is Sweating a Joint?
“Sweating” and “soldering” are synonymous terms used to describe the process of using heat and solder to join copper pipe and fittings to one another. You can tell your neighbor you’re heading downstairs to “solder” a joint—but “sweating” makes you sound much more worldly and DIY hip.
Sweating involves multiple steps, each of which must be done correctly to ensure a water-tight joint. Both the pipe and fitting must be deburred (remove any sharp edges, metal or shavings resulting from cutting the pipe), then cleaned with a wire brush and emery sandpaper to remove dirt, oils, burrs and other substances that could get in the way of creating a tight fit. Flux is applied to both parts, then the two pieces are snugged tightly together. The joint is heated, usually with propane or MAPP gas then, once the proper temperature is reached, solder is applied to the perimeter of the joint. If you’ve done your prep work correctly, the solder is sucked into the joint creating a long-lasting water-tight seam.
Professional plumbers—because they sweat hundreds of joints in a year—are fast, precise and rarely err. But with practice, even confident beginners can “sweat a joint.” There are many YouTube videos describing this process in detail. Plus, this article has more details about how to solder a copper pipe, as does this article.