what happened was when you welded it expanded the metal causing a high spot and the panel formed a oil can.
now to fix it properly with a heat shrink.
you need to hit the oil can area UP and then find the highest point in the panel. a lot of times it will not be in the oil can area.
heat this spot up with a torch until it's cherry red, note don't heat up a area any bigger that the size of hammer head, i my myself stick with about 1/2".
now hold a flat dolly behind the heated area. strike the panel around the flat spot with the force of the blows towards the red spot. hit it about 6 times around the circumference of the spot. then strike the heated spot directly forcing the metal you moved towards the heated spot to to shrink.
now cool the heated spot and the oil can should be gone.
you just did a heat shrink.
if not find the next highest spot and repeat the above steps.
the most important thing to remember is heat shrink the high spot of the metal. a lot of times it won't even be in the oil can area it will be next to it.
i myself don't like shrinking hammers and things like that. if used improperly they can make more problems than what you had.