if you want to read a comprehensive study of the problem, go to: http://www.dt.navy.mil/cnsm/lead_01.html
Page through this article by the US Navy's Curator of ship models. These are often made of lead, too. Basically, the article says that lead models need to have air circulating around them, and be kept away from contact with wood. The article says that there is no known cure, and: "It is important to recognize that the formed lead carbonate is not just a substance clinging to the surface of a casting, it is the surface of the casting transformed to powder. For practical purposes, a portion of the lead is gone and lead carbonate is left in its place."
I have about 700 old ship recognition training models, and I have found that I can clean up old lead castings by soaking them for a few minutes in vinegar. This loosens up all of the corrosion, and with some soap, water, and a toothbrush I can restore a lead model to a shiny new condition. Of course, then it becomes necessary to seal the model in an attempt to stave off further oxidation.
Hope this helps! -- Bryan