Step 4 · Apply Soldering Flux
Using a small flux brush, apply a thin, even layer of soldering flux to the cleaned outer surface of the pipe end. Then, apply another thin layer of flux to the cleaned inner surface of the fitting. The flux is a chemical cleaner that prevents oxidation when the metal is heated and helps the solder flow evenly into the joint. Do not use an excessive amount of flux, as it can run inside the pipe and corrode the copper over time.