Hey Jonathan,
This what I did for my stormtrooper commander...
1. I drew the figure on paper and marked where I wanted the damage.
2. I then primed and undercoated my armour.
3. I marked on the armour all the areas I planned to damage with a 6B pencil and painted over those areas with a silver paint,(for the indented areas, I used a heat gun on the area then gently made the indentations with some jagged metal)
4. Once that was dry, I used masking fluid over the silver, let it dry and painted it with a semi gloss white and humbrol 15 Midnight blue or equivalent for the rest.
5. I allowed it to dry for 4 days
6. I lightly sanded the armour initially until it took on an almost matt finish.
7. I then used a very course sandpaper on areas to put in some deep scratches.
8. I then used a light black acrylic wash to cover the armour, making sure it got into the scratches, ridges and hollows (you can also use the 6B graphite pencil or powder if you'd prefer).
9. Once the acrylic wash has dried, you can simply peel off the latex masking fluid, which leaves the nice silver scratches and indentations.
10. Lastly, I used some fine sand paper to gradually remove the remainder of the acrylic wash.
I hope this helps
Rick