I was looking through some pictures of the 747 particularly this one :
Photo © Ken Rose
And I have noticed that not only on that one but other 747's as well, there is a large dirt build up by the rudder, I think its the hydraulic acid- not sure? I have noticed this on many 747s and I always wondered why. I know the 747in the picture is unfortunately no longer with us, but any more info on why this is common - even along the wings too would be great!