A method of joining metal sections (12, 14), comprising the steps of: providing a first section having first substantially L-shaped groove face; providing a second section having a second substantially L-shaped groove face; aligning the first groove face of the first section against the second groove face of the second section to form a narrow weld groove in which the metal sections form a butt joint along an interface at a root having a centerline and a groove above the root wherein the first groove face and the second groove face are separated by a predetermined distance above the butt joint; and characterized by the following steps: providing a first arc welding device and aligning the arc welding device against the root on a side of the butt joint opposite the groove and offset from a first side of the root centerline; providing a second arc welding device and aligning the second arc welding device against the root within the groove and offset from a second, opposite side of the root centerline; providing a laser welding apparatus and aligning the laser welding apparatus so that the laser beam is aligned with the root centerline and focused at the interface; simultaneously applying power to the first arc welding device to strike an arc, the second arc welding device to strike an arc and to the laser welding apparatus, to establish a stable keyhole while forming a common pool of molten metal at the root without burning through the root, while supplying filler metal with the two arc welding devices (40,46); and moving the first arc welding device, the second arc welding device and the laser welding apparatus simultaneously relative to the root to form a full penetration weld along a length of the root pass. providing a first section having first substantially L-shaped groove face; providing a second section having a second substantially L-shaped groove face; aligning the first groove face of the first section against the second groove face of the second section to form a narrow weld groove in which the metal sections form a butt joint along an interface at a root having a centerline and a groove above the root wherein the first groove face and the second groove face are separated by a predetermined distance above the butt joint; and characterized by the following steps: providing a first arc welding device and aligning the arc welding device against the root on a side of the butt joint opposite the groove and offset from a first side of the root centerline; providing a second arc welding device and aligning the second arc welding device against the root within the groove and offset from a second, opposite side of the root centerline; providing a laser welding apparatus and aligning the laser welding apparatus so that the laser beam is aligned with the root centerline and focused at the interface; simultaneously applying power to the first arc welding device to strike an arc, the second arc welding device to strike an arc and to the laser welding apparatus, to establish a stable keyhole while forming a common pool of molten metal at the root without burning through the root, while supplying filler metal with the two arc welding devices (40,46); and moving the first arc welding device, the second arc welding device and the laser welding apparatus simultaneously relative to the root to form a full penetration weld along a length of the root pass.