So in the several versions I ran into a bug where when I try to use the vehicle welding rig to repair an item and this message comes up. “Lost tool used for long repair” So while I can use it for crafting it will not let me repair it.
I can confirm this bug in 0.C-8539-g59a306e. Using the integrated soldering iron to repair equipment fails with the message DeWolf stated. Regular soldering iron’s still work as usual, and fixing cars with the vehicle mounted welding rig still works.