Figuring out how to add battle damage to metal armour (or unpainted armour) can be confusing for some folks. The main reason is that the method we often use to represent battle damage on normal (painted) armour will not work here. We have to alter our normal method slightly.
The process remains the same as it would be for adding damage on painted armour. We add the light color first and then offset the dark color (slightly higher for overhead lighting) on top of the previous color. This gives the impression that there is a chip or dent in the surface of the armour.
It's another faux painting trick we use to convince the viewer without actually having to damage our models. By leaving that very thin line of white along the lower edge, it simulates the overhead light being reflected off the lower portion or lip of the battle damage.
The trick to this whole thing
The key to this whole thing is in the two colors and the sequence.
Instead of a lighter shade of our armour color, we're going to use white. We want the thin line that's left over to pop and it's going to take white over the metal to make it show up. Our other color is black. You could use a dark grey color if you wanted, but make sure it's almost black since you don't want it being confused for just another shadow or a stray mark on the model.
The first color to go down needs to be the white. By putting the white down first, you can add the black right up the edge of it leaving only the tiniest sliver showing. It's much easier to get a thin line of white in this manner than it is to paint the black first and then try and add the white along the remaining edge.
You really do need that sharp, irregular edge of white to make this look convincing.
And that's it, simple as that for adding damage to plain metal. Now there are things you can do to dress this up even more, but this is a great start and will work perfectly for tabletop models. And remember, when it comes to adding battle damage on a model, less is more!
Make sure to check out these posts as they might help:
Weathering: Getting a dirty metal look
Using the same faux trick to paint hair
The right way to start your weathering