Process for making a panel structure by superplastic forming from a pair of face sheets disposed on opposite sides of a core sheet, the core sheet being bonded as by welding to the face sheets along a plurality of spaced weld areas, the core sheet and face sheets having superplastic characteristics, which comprises placing a restraining sheet of superplastic material in contact with one of the face sheets and subjecting the forming pack of the face sheets and core sheet, together with the restraining sheet, to superplastic forming in a die while applying a back pressure against the restraining sheet. The back pressure is lower than the internal pressure and maintains the restraining sheet in contact with the adjacent face sheet while maintaining the other face sheet in contact with a surface of the die. After superplastic forming the resulting panel structure formed of a core web and attached face sheets, and the restraining sheet, are removed from the die and the restraining sheet is separated from the panel structure. The process results in a panel structure with flat face sheets, and without grooves or wrinkles at the areas of the bonds between the face sheets and core sheet.