Nikon D600 has some serious sensor dust issue since it released. Today, we got official responded on this issue from Nikon USA:
Measures to reduce effects of dust or foreign matter are optimized for each model. Therefore, the dust reduction system’s internal mechanism varies with each model,” the Nikon statement reads. “If the effects of dust or foreign matter on photographs become bothersome, customers are encouraged to consult their local Nikon service center.
The Nikon USA simply said if your D600 has dust issue, just go to your local Nikon service center for help.
via: imaging resource