Do this. Grab a oxy/acet torch and heat the pipe up to a cherry red in the area that you are going to weld. Doing so will clean the metal perfectly that you are about to weld. I didn't do this and let me tell you the welds look like chit but held. It took me about 2 hours to put 2 flowmaster mufflers on. I was too busy filling holes because the metal would not stick.