Donatelo:
Weathering is not my speacilty, but I have been reading atricles and experimanting a little with it. On another site there is a guy who wrote that since the older BB's were coal burning that he used a pencil lead scraped on the verticle surfaces to represent the stain the coal left from the stacks.
He would scrape the verticle details with the lead then smudge it with his fingers or a brush in a downward motion. This give the ship the look of coal being washed off by the crew.
 http:www.modelwarships.com
Click the how to link and the aricle is there
I posted the exact link somewhere on the warship forum thread but I can't find it. 
 
  Hope this helps and good luck.