Currently, the Soldering iron, as far as I’m aware, relies purely on Mechanics skill when it comes to repairing/reinforcing metal objects. This is pretty weird and inconvenient, given that the majority of metal objects that are likely to get damaged come from, or at least require, the Fabrication tree.
As it stands, it’s possible to use your level 9 fabrication skill to smith a rapier, and be completely unable to repair even minor dents to it until you’ve practiced working on cars some more. Soldering irons using Mechanics seems to be a holdover from back before the Fabrication skill was a thing.