Water Woes

• Water is seldom an issue when you are working with pipes in new construction that have never had water in them. When tapping into an existing line that has been previously used, water can be a major issue. It's impossible to solder a fitting to a copper pipe with water in it. Allow every drop inside to run out before attempting to sweat a fitting to an existing pipe.
  
  

Pipe Cleaning

• The solder used to sweat pipes is designed to adhere chemically to pure copper. It won't stick to dust, dirt or corrosion accumulated on the pipe or to the dull brown oxidation that naturally occurs when the pipe is exposed to the air. Use an emery cloth or stainless steel wire brush to shine the area of both the pipe and pipe fitting to expose pure copper in the areas the solder is to adhere.
  
  

Soldering Paste

• When a copper pipe or pipe fitting is heated hot enough for it to melt solder, oxidation will occur in a few seconds. Use soldering paste as a flux to keep the oxidation process from occurring so the solder will stick. Apply flux to both the pipe and the interior of the pipe fitting being soldered.
  
  

Environmental Issues

• The heat source most often used to solder copper supply lines is supplied by an open flame from a propane torch. Be sure the area you are using the torch is free from any flammable fumes or liquids. Insert a metal heat shield between the pipe and any wood or other surface that could be ignited with the torch while soldering the pipes.