Methods for welding sandwich structures together, especially vehicle structures, where the structures include two opposing face sheets with a core, such as a micro-truss core, therebetween. In one embodiment, two sandwich structures are butt welded together. A cooling fluid barrier is provided at an end of the core between the face sheets to prevent cooling fluid from entering the core during the welding process, where the cooling barrier can be a micro-truss structure and can include cooling fluid channels. A welding insert is inserted into the end of one of the structures adjacent to the cooling barrier, and is welded to the other structure to secure the structures together.