[NefCanuck]

Not to doubt what you've been told Nik, but I was told that with more recent cars the problem was that drilling holes to spray behind the panels actually caused rust to form at these new entryways. As it was described to me it was because the drilling compromised the coating on the steel used and introduced contaminants that would eventually filter down to the lowest point in the car and together with whatever moisture there was, start causing rust. (I'm not sure but I think the anti corrosion treatment at the factory is a zinc based product applied to teh steel during the manufacture)

NefCanuck