There is also a difference between areas where rust and grime have been deposited and where paint has gotten water under it and rusted through. Going back and forth with the base colors and rust gives the impression that the rust has built up over time. I would also advise against heavily weathering and rusting the upper works. Deck crews are usually tasked to chasing those areas down and keeping them as shipshape as possible depending on service conditions- even under way. There are exceptions, of course, especially in northern areas of the Atlantic and Pacific oceans.
Faded paint is common but rust is usually dealt with promptly.
They can't get at the hull until they anchor somewhere and can put a chair over the side. If you are doing a full-hull ship, don't forget the barnacles and paint fading there, too. I had a hard drive crash and lost the URL and pics of the Spruance class DD that got rammed, but there were excellent references for hull weathering on that site. I apply them a little heavier along the waterline and in depressions to add depth. Remember to mask the hull above where the water normally reaches.