Here’s an example of a “rod through the filler neck” where I needed to keep the paint in tact....
The side of that tank was creased top to bottom worse than the OP’s. It’s not a straight rod, and it has a spoon like tip. You run it up and down the crease in a similar action as using an English wheel. You can also work a dolly on the outside.
Like I mentioned before, which you may have missed, the point of using heat is for an overstretched panel that you want to shrink back. I’m not sure if these pics will make sense, but this is from the front end of a Plymouth Roadrunner that was pinned between two trees, so both sides had a nasty crease that sunk in several inches of depth. The creases ran from where the bumper meets the fender to about 6 inches or so above the wheel opening...
I used a rosebud to heat the panel in the second pic because after hammer and dolly, it bulged out too far. A rosebud allows you to spread the heat and not get it too focused. How hot it gets has a lot to do with your gas mix adjustments.
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