Be VERY careful with that air. Go not above 5 psi. The power of constrained air to deform panels is not to be understated.
Maybe do the inflating with one of the rubber pointed air chucks. And hold it with your hand. Thus you could let off at any sign of failing, or if your hand gets tired!
I have a gas tank that was used on a doodlebug. It has a large "crease" across the top where a cable must have been wrapped and left it's imprint during it's workaday life. I would dearly love to remove that crease - the tank is in very good condition other than that and I expect will still hold gas.
I've thought of different ways to remove that crease, including separating the halves and hammering it out from the inside, or possibly nflating the tank to the point where it "bumps" out. But I dare not. Do the arithmetic. 15 psi x 10 inches deep by 30 inches wide - that's a lot of force. And welded according to a submerged arc procedure that today would be considered not to current weld thinking.
So instead I'll simply fill the crunch with bondo, and use the tank minus one gallon capacity.
No biggie.
Joe K