Yes, applying plumbing flux is an absolute requirement when soldering (sweating) copper pipes and fittings. Flux is a chemical cleaning agent that chemically removes residual surface oxides, prevents new oxidation from forming when the copper is heated by the torch, and promotes the capillary action that draws molten solder smoothly into the joint gap. Without flux, solder will pool up on the exterior instead of flowing uniformly into the connection, causing a leaky joint. Verification: Clean copper pipe ends with sandpaper and brush on a thin layer of water-soluble flux before heating.
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