+1 for GMC's comments.
I had a YZ125C tank that was pretty rough. I learned the hard way about them spreading under pressure and then learned the hard way about the pressure collapsing the frame tunnel...
Eventually, I had a piece of 4x2" timber wrapped in rags in the tunnel, and held the lower seam in the big vice. Then I cranked the pressure up...
In hindsight, if it had let go at 100psi, it probably would have killed me, but it didn't, so that ended well.
What did happen was that the bigger, shallower dents were made smaller, and the smaller, sharper dents completely ignored me.
So then I bogged it.