Originally posted by markp27 I know some guys fill the exhaust with water, blocking one end, and then place it in the freezer - the expanding nature of water when it freezes pushes out the "dents". But keep a careful eye on the pipe in the freezer - don't just leave it there or else it may split.
You may want to put cable ties wrapping around the pipe to prevent the pipe from over expanding in the freezer. Woop ! I hope it's not too late.