The trick he explained works if you have a horizontal copper pipe that’s opened up while the water is off. As you try to solder it water gets drawn to the heat and you’ll never get the pipe hot enough to get a good bond. The white part of Wonder Bread or something like it works well because it’s almost all starch that will dissolve in the water afterwards. They sell manufactured plumber’s bread, but it’s basically the same thing. As someone already noted, you’re supposed to bleed your lines before you put your fixtures in so that you don’t get any debris stuck in your aerator screens.
