I do ORM master also. Thanks for the tip. Saves me manually doing it. 
In still of the opinion that this whole subject is basically flawed.
You have metal. Period.
you have non metals. Period.
To combine the two, why not just use material layers at this point?
Take the scraped chisel from the page before. Say you are super lazy and unwrapped it vert badly, and just painted the texture on top of it. You have one unique material even though it clearly calls for at least two even 3 (the handle, the marking on the handle, the blade/metal).
at this point your laziness in making the unwrap/texture means you need to put in some extra work on the material. Create an alpha map to fade between wood and metal.
and another one to fade between shiny metal and oxidized metal.
lerp the materials, or better yet use layers, using those alpha maps and you sort of solve the problem.
This applies to the rusted metal as well, you can have a layer that is metallic, and simply overlay a non metallic transparency/masked effect onto it that looks like rust.
You’d be had pressed to find shiny yet rusty metal in the real world since oxygen and oxidation are a thing, but that doesn’t mean it can’t happen… you could have scrubbed rust off only partially.
While I can see how having a “metal map” would seem to be an easier solution, I think that even remaking the material manually by having the 2 materials end in make material from attribute nodes and mixing it up with a layer blend driven by a custom alpha map would end up being a way better overall solution. At least, your metal would always be metallic…
The metalness setup is mostly one to save memory. Most dielectric materials have a white specular color, no color is needed for the reflective part, conductive materials (metals) on the other hand have a black diffuse but a colored specular. So the albedo color is used for the reflection of metals because it isn’t used for diffuse. This is a simplification and there are things that don’t fit into either of these categories, but for most common objects this works well.
For something like that chisel consisting of wood, rust and metal (let’s just assume that the front part is mostly metal with a bit of rust) all of those can be represented well with an albedo, normal, roughness, ambient occlusion and metal texture. Pack those channels and you can use that model with two textures and a material that can be used for many different models (with different textures packaged in the same way).
The chisels I used when I was a kid just were always like that, they were rusty near the grip, but the part you use to carve into the wood with was always free of rust, because of how they are used.
Well, i agree with your remark, the preview doesnt either look right and i canùt differentiate what is wood and metallic, even rusted, oxided and if i check this asset today in the megascan library.
We also have noticed some textures with a double gamma correction, quality is not always as it should be