EarthQuake's Simple Metal Tutorial

This is just a real quick example to share some basic points.

Its generally really important to have most of the detail in your specular map, scratches and scuffs should end up here(maybe SLIGHTLY in your diffuse as well) to create convincing metal materials. And to remember that things like rust, or dust/dirt(which may be lighter in your diffuse) should be darker in your spec.

Its good to have some sort of environment map, in marmoset our lighting is image-based which means we always have a enviro(cube) map, we just adjust the blurriness/sharpness of the cube to get different levels of specularity sharpness(often referred to as glossiness) and it can be nice to have an extra map for this if your engine supports it, because you may end up with skin(low gloss) and metal/leather/etc all in the same texture.

And finally you also want to have some detail that is unique to your diffuse, stains, etc that really don't exist in your spec, just for a nice bit of variation. I see a lot of people who create specular maps that are basically just copies of their diffuse, desaturated with some level/brightness changes. Really for a lot of materials you want each of your spec/diffuse maps to have a good bit of unique detail.