This video tutorial is in the Autos, Motorcycles & Planes category which will show you how to heat shrink metal with oxy/acetylene. A stud nail gun or a heat inductor may also be used for heat shrinking. Metal popping in and out is called oil can effect, which is caused by stretched metal. You got to shrink that part of the metal to remove this defect. For doing this, first you set up everything correctly, light the flame and adjust the gas flow so that the flame is of blue color. Heat an area of the metal surface the size of a dime and then lightly tap the area with a body hammer. Repeat this process till the oil can effect is removed from the entire metal surface.