What I mean is that you paint a thin line - about the width of your brush - of quite thin paint around the edge of each plate, the rims of the shoulder pads and so on. You want the paint to be pretty thin because that will make it more translucent.
Finally use the Space Wolves Grey and run the edge of your brush around the corners of the plates so that the very edges of the plates are outlined. Make sure you don't have too much paint on your brush otherwise it will go everywhere, just run the side of the brush along the edges of the plates and you'll end up with a nice neat highlight.