Now, technically the rules allow me to be fairly vague on what my item would protect me against, and the duster is listed as being either Armor 1 (Lore 4 halved to 2 for persistence, halved against to grant an armor bonus) or a persistent fair (+2) spirit block vs. physical attacks (Lore 4 halved to 2 for persistence). What if it's for something more specific than just 'physical attacks,' such as how I'm intending this to only affect physical attacks be primarily-metal weapons? Is there any difference in the process, here? I'm limiting it to fit Leo's Wizard in Training (Geomancer) aspect, since an Earth-based effect seems like it would relate to magnetism and thus create a repellent effect on metal.
On the other hand, I could always just make it more of a "tough as granite" effect to protect me, if it's easier to work with.