First off, you have to choose where to put damage on your mini, and what kind of pattern to follow. You are looking for that zen combination of realistic looking effects, and what will support your mini’s composition. For realism, the areas that are most susceptible to damage are the hard edges of the mini. Any edge of metal plates are more likely to get scuffed. Also, there is more likely going to be some damage on the lower parts of the mini (near tank treads or feet) as these areas are likely to get worn and scuffed on rocks and terrain.
Beyond that, however, it is usually good to have some damage in other areas too. This can help your composition quite a bit, and is easily justified on war machines and trooper armor.
I almost always start out by painting up the areas I want damaged in black. This allows for two things- first off, it suggests that the damaged armor was painted using a primer, and that it is showing through a little in the damaged areas. It also gives you some strong black-lining for the damaged areas themselves- and the black lining helps the battle damage to “pop.”
I sometimes use a small sponge of a piece of foam to apply the black. This allows for a very scattered and random pattern for the damage to follow, which helps the damage look more realistic.
The second step I usually take is to put on a background metal for the damage itself. I find that using a little bit of a darker bronze-ish color is best for this. Good examples include Citadel Tin Bits, P3 Brazen Brass or almost any bronze color mixed with P3 Brown Ink. This will give you a good background for your silver damage- it helps to make the areas look more worn, slightly rusty without going too far.
By the next step, you are ready to apply some silver to your areas. You’ll want to thin your paint down a little, so that the bronze color underneath can show through. If you have large enough areas, you might want to consider layering up from darker silvers up to lighter ones. Another option is to mix some of your base bronze with your silver as you layer up.
I often go all the way up to Citadel Mithril Silver for the damage. The bright silver gives the area a more scratched up look while at the same time pushing the silver forward for composition reasons.
Now you could be finished here, but you don’t have to be. I often apply thin brown or blue washes over minis that are battle damaged- making them look either worn or dirty. If you are applying any washes like this, then make sure to get the damaged areas. A wash can add a nice touch to your damaged armor.
One last note on metal rivets. If I want rivets to stand out as metallic pieces, I usually paint them up using the same methods mentioned above- often I leave out the black. Painting rivets like this makes them look like worn raw metal- and they should match your battle damage well.