Step 4 · Apply Flux
Using a small flux brush, apply a thin, even layer of water-soluble paste flux to the cleaned outside surface of the pipe end. Also apply a thin layer of flux to the cleaned inside surface of the fitting. Flux is a chemical agent that serves two purposes: it continues to clean the copper when heated and it prevents the surface from re-oxidizing as you apply the torch, which allows the solder to flow evenly into the joint. Do not use an excessive amount; a light, complete coating is all that is needed. Avoid getting flux inside the pipe where it could contaminate the water system.