Cleaning copper prior to joining pieces using a soldering process is important. Impurities on the metal will obstruct the molten solder from adhering properly. This will create a fault in the joint. Whether the copper is wires, pipes or full sheet material, the connection to form that joint must be as clean and as bright as possible.
Soldering wires together forms a permanent connection. That connection relies solely on how well the solder will flow between each wire and create a conductive path for electrical power. Wires that are protected from oxidation or residue will need no cleaning. These types of wires will be protected by a layer of plastic insulation. Stripping that insulation back and then twisting the wires around each other will form a good joint. Wires that show signs of oxidation and corrosion will have to be cleaned. In most cases using a piece of small grit sand paper, 300 grit to 600 grit, or a piece of emery cloth will brighten the wires for a properly soldered joint. The higher the grit rating on the back of a piece of sand paper, the smaller the sanding particles.
Pipe connections for plumbing or perhaps as a hobbies project will have two basic methods for cleaning. The first method is to use a piece of sand paper or emery cloth, as described above, to polish the outside surfaces to a bright appearance. Internal pipefittings will need the use of a steel brush slightly larger in diameter than the opening of the female fitting. The brush is inserted into the fitting and rotated in either a clockwise or counter clockwise direction. This will clean the surface of any built up obstruction to solder flow. Typically a cleaning rosin flux is then brushed onto both surfaces of the pipe joint. Heat from the propane torch will create a fluid that not only cleans the joints, but allows the solder to flow into the mating surfaces.
Unlike pipe or wires, sheets of copper may still contain a layer of manufacturing oils on the surface. The oil is used for keeping the material lubricated during the rolling process. In most cases, the residue of oil is kept in place to reduce the oxidation that may occur while the material is in transport or storage. Removing this layer of oil is vital in making a proper soldered joint. Lines should be drawn on the material that will indicate the joining line. Cleaning the sheets should only take place where the solder is going to be applied. Removing the oils from the entire sheet of copper will accelerate the oxidation process and turn the bright copper sheet to a green color. Use a clean cloth that is wetted with acetone. Apply the rag to the joint area. Turn the rag to a clean side as the oil builds up on the cloth. Add more acetone to the cloth as needed. Follow all safety instructions on the acetones label. Acetone is highly flammable. As a final treatment, slightly scour the surfaces with emery cloth. This will brighten the copper and remove any residue that could be left from the acetone cleaning.