What you need to do is shrink the metal .
take your torch and make a small flame and heat and area to cherry red to a size of a dime , and cool it quick with a watered down rag , this will shrink and tighten the metal , you will need to lightly hammer and dolly to help bring the metal straight, as the metal may swell or suck in .
You may have to do this several times in different areas to get the metal to stop the tin canning .

This should work .

SprayTech