It's not normal for 'nasty stuff' to come out.
Do you drain the gascolator first? If you do, you can drag nasty stuff down from the tanks into the pipes, and this can reach the engine at embarrasing times. When you drain the wing tanks you don't see the muck because it's down in the pipes from when you drained below. Sort of poorly explained but you get the picture. You should always drain the tanks first.