Here's one that will show you some ways to do the PBR painting directly in Blender. This one is for ICE material, but the techniques work for just about any texture. Metallic maps are usually just greyscale colors to indicate how metallic the object will be. A steel material will be completely white. The detail comes from Normal, Roughness, Specular, and other types of maps that complement the type of material. Metallic objects are either metal or not, so typically just black or white areas to indicate which areas are metallic and which are not. Sometimes people use scales of gray to add some metallicity to certain objects, which increases reflections, but it's not technically correct to do so.

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