The standard calls for a hard coat. It says "Smooth-haired, short, hard, thick and close lying. Invisible grey undercoat on the neck permissible." The coat down the back specifically should have a bit of wiry texture in blacks and reds. Fawns and Blues may feel more wirey, as the melanin isn't evenly distributed through the hair shaft. These lumps of melanin leaves these areas of the strands brittle, and easier to break. I see you obviously, have a black dog, but thought I would mention it for future readers that may find the thread.
I think the sardines mentioned before can definitely be helpful. I mix one or two into my boys meals here and there, not necessarily for the coat, but I am sure it doesn't hurt.