Step 4 · Clean the Pipe and Fitting
This is the most critical step for a successful joint. Any contaminants like dirt or oil will prevent the solder from adhering. Use a strip of emery cloth or fine-grit sandpaper to thoroughly clean the outside of the pipe end that will go into the fitting. Sand it until the copper is bright and shiny, like a new penny, for about 1 inch from the end. Next, use a wire fitting brush to clean the inside of the fitting. Insert the brush and twist it several times until the inside surface is also bright and shiny. Do not touch the cleaned surfaces with your fingers, as oils from your skin can contaminate the joint.