Apply Soldering Flux

What happens here

After sanding, apply a thin, even layer of soldering flux to both the cleaned exterior of the copper pipe end and the interior socket of the copper fitting. Ensure complete coverage of all surfaces that will be part of the joint. The flux acts as a cleaning agent when heated, removing any remaining oxides and allowing the molten solder to flow evenly and completely into the capillary space between the pipe and the fitting. Carefully wipe away any significant excess flux from the outside of the joint to prevent discoloration or hard-to-clean residue after soldering.

Goal: Prepare surfaces for effective solder flow and strong adhesion.