On open structure like a wing and on most sheeted structure also you want to tack down the perimeter real well and then use the gun to shrink the center down evenly. If a bubble appears in sheeted areas a pin can be used to vent the trapped air. After the gun, iron the covering down to the structure to bond the covering to the wood. On open structure, try to leave very tiny holes in the structure to allow trapped air to escape. Usually there is already plenty of holes in wing structure