Some old threads on caulking and sealing roofs.... Gard seems to have alot of info about properly cleaning and sealing steel and aluminum surfaces....
http://www.horsetrailerworld.com/forum/thread-view.asp?threadid=11834
http://www.horsetrailerworld.com/forum/thread-view.asp?threadid=12206
http://www.horsetrailerworld.com/forum/thread-view.asp?threadid=12713