Step 5 · Apply Flux to Surfaces

Using a small flux brush, apply a thin, even layer of lead-free soldering flux to the cleaned outer surface of the pipe end. Also apply a thin, even layer to the cleaned inner surface of the fitting. The flux serves two purposes: it chemically cleans away any remaining trace oxidation and prevents new oxidation from forming as you heat the metal. It also acts as a wetting agent, helping the molten solder flow easily into the joint through capillary action.