As queen_annes_revenge says, small very fine lines on prominent areas which would receive wear are a very good way of simulating damage.
If it's on a corner, I tend to do a bit of a splodge of the highlight colour (in this case it would be Bering Blue or Artic Blue if I was painting black armour, depending on the depth of the scratch) and then draw it out across the surface. I then use the base colour (black or Abyssal Blue) to 'fill in' the centre of the scratch, giving the illusion of chipped paint.
This guide by Giovanni Di Lena is what I follow and probably illustrates it better than I can:
http://figurementors.com/painting-black-power-armour-by-giovanni-di-lena