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Use sandpaper, steel wool, or anything else abrasive to clean away any grime or dirt from any areas of your pipes and fittings that need to be soldered. This will let your solder hold more tightly to the pipe and result in a more sturdy connection.  You should only need to clean around 1 to 2 inches (2.5 to 5.1 cm) of the piping to give you room to solder easily. If you're using a larger pipe fitting, you may need to clean more to ensure the point of overlap between the pipe and the fitting is prepared for soldering. Use 120-grit sandpaper or grade 1 steel wool for cleaning your pipe fittings. This should be enough to strip away dirt and grime without damaging the pipes themselves.  If you plan on cutting and soldering a lot of pipes, you might benefit from a specialised pipe cleaner. This will let you more easily scrub away any impurities on the inside and outside of your pipes. Ask at your local hardware store or look online to find one that suits your needs. Use a small paintbrush to cover the ends of your pipes and the insides of your pipe fittings with a thin layer of flux. Look around the edges of the pipe for any large lumps of flux and brush them away. Any type of flux that you have will work when soldering pipes together. Acid flux will be more corrosive, but may damage pipes that are too fragile. Tinning flux will help hold your pipes together more strongly, but may be more difficult to find or more expensive. If you're unsure, ask at your local hardware store for advice on the best type of flux to use for the job you're working on. Hold the two sections you need to connect together in each hand, keeping them off of your work surface to prevent any flux from rubbing off. Push the two ends together securely until they are fully connected. Wipe away any flux that comes off with a clean paintbrush. You should connect a section of a few pipes together before you begin to solder them. Working one at a time may be very slow, and trying to deal with a long section all at once may be very difficult! Work with a few pieces at a time. Heat will make the metal expand, so you should always heat the larger sections of your pipes up before the smaller sections. Hold a soldering iron to the pipe, or use a blowtorch to heat the metal up until the soldering flux melts into the joint and begins to bubble slightly. When the flux is just starting to bubble and the pipe is hot, press the tip of the solder to the opposite side as you are holding the blowtorch. The solder should melt immediately and work into the joint to hold it together. Pull the torch away and quickly run the solder around the circumference of the joint to completely seal it.  You should only need to use around 3⁄4 inch (1.9 cm) of the solder to completely seal the pipe. If the solder doesn't run into the joint and forms small beads that roll away from it instead, you might have burnt the flux or may not have cleaned the pipe properly. Wait until the pipes are cooled, disconnect them and start again. It's easier to reach around the back side of the pipe if you bend a small hook in the solder wire before you begin soldering. Grip the solder far enough away from the tip that you won't risk melted solder getting on your hands. For extra safety, wear fire-resistant gloves as you work. Once the pipes have cooled a little, look around the edges of the joints to make sure they are evenly coated with solder. If you notice any sections without solder, apply a small amount of flux on the area and let it melt with the residual heat of the pipes. Use the blowtorch to heat the pipe up again and apply a small amount of solder to the bare area. It may take a while to get a good feel for when pipes are completely soldered. As a general rule of thumb, as long as there is a thin, silvery trim around the edges of your fittings, the pipe should be fully soldered.

Summary:
Clean the area that you'll be soldering. Apply flux to the outside to your pipes. Connect the pipes and fittings together. Heat the fitting or female connector up with a soldering iron or blowtorch. Press the solder to the opposite side of the pipe. Examine the joints to make sure they are completely soldered.