Just to add my two-pennyworth, Tow'd,
the guys who pressure-form sheet-metal for small-scale production strongly advise you use liquid pressure, normally water, for the reasons R Pope has stated. Water does not compress under pressure, so movement is incremental, whereas air does, and can store massive amounts of energy, then when something gives - it can release it in one go; think of an air-rifle.
These guys use liquid to hand-form 'bike fuel tanks, two-stroke exhausts, etc.; it's more controllable.
ps, I've had success in the past with diesel and kerosene mix. You sure ain't spoiled for choice
Len