In addition to general rust and scuffs on the hull one of the most noticeable items of weathering is soot from the stack. The boilers would typically burn pretty cleanly to avoid making smoke, but every day or so you need to blow tubes (basically using steam to blow the soot off of the boiler tubes and out the stack). This leaves flecks of black soot all over anything downwind. In 1/700 scale effect of the flecks scattered on the deck would not be visible, but the top of the signal mast on the flying bridge should be stained black starting several feet below the top of the stack. The top of the charley noble (galley stack) should have similar staining due to the coal smoke.
One other weathering item is staining from the discharges located above the waterline (if you are doing a fully loaded ship they are just barely above it). There are several located on the either side amidships, as well as two aft on the port side under the forward end of the afterhouse. Most of these discharge liquids from either the accommodation drains (deck drains, sinks, and showers) or toilets. There are also a few discharges forward, but they seldom would have anything coming out of them.
One less noticeable (at least in 1/700) item is grease from the cargo runners. They are slushed with a very messy grease that has a habit of leaving black streaks on anything the wire touches, particularly on the rails (when lowering the cargo hook to the pier the runner from the offshore winch usually rubs across the top of the rail).
Regards,
Chris Friedenbach
Crewmember, SS Jeremiah O'Brien