Step 4 · Clean the Pipe and Fitting
This is the most critical step for a successful joint. Solder will not adhere to oxidized or dirty copper. Use a strip of sand cloth to vigorously clean the outside of the pipe end, going about 1 inch up from the end. The copper should be shiny and bright, like a new penny. Next, use a fitting brush (wire brush) to clean the inside of the copper fitting until it is also bright and shiny. Do not touch the cleaned surfaces with your fingers, as oils from your skin can interfere with the solder.