Sweating Copper Pipe
Getting a good solder joint is as much about cleaning the pipe as it is about melting solder. Use a plumber’s pipe cleaning brush or sandpaper to clean the oxidation off the outside of the pipe and the inside of the fitting. They should be shiny like a new penny. Coat the inside of the fitting and the outside of the pipe with lead-free flux, then slip the pipe into the fitting. Being careful not to set anything on fire with the open flame, heat the fitting with a propane or MAPP gas torch (The latter burns hotter and is therefore easier to use). When the copper is hot enough to melt the solder, run it around the joint. The heat will pull the molten solder into the joint. Let it cool naturally until the solder solidifies, then wipe any excess flux off the pipe with a rag.