I have found that underbody rust can occur without the main body rusting badly. The quantity of underbody rust seems to depend on the frequency of snow and salting. Most winters in Montreal and Toronto (I have spent many in both), the roads spend a lot of time dry, in spite of the quantity of snow we get. This past winter, however, it snowed most days for a couple of months. As a result, the roads were constantly wet with salty water. This is what kills the underbody. In 1995, I moved to a little resort town north of Vancouver. I took with me a car I had owned since new. It was a 1988, and before I left, I rebuilt EVERYTHING on it, including re-rustproofing the underbody after a thorough cleaning. It was in perfect condition. Well, in that particular resort town, it snows almost every day in the winter. In 1997, I moved to Vancouver and sold the car for peanuts. The underbody was gone. Completely rusted out. The rear crossmember needed major work just to keep it on the car. The main body was still quite presentable, and the engine had only 30,000km on it since I rebuilt it. I just didnt have the heart to do any more work on it.