Maintenance, not defect
And its not the water, it's the grit and salts that do that. If you rinse the car off well regularly, including underneath, the engine, tailpipes, and behind the license plate, you won't have this problem. The water will evaporate, the dirt does not.
About a month ago, while giving my 2007 CX-9 a wax job, I noticed the dreaded tailgate rust just starting to peek out from under the corners of the plastic trim piece that's located in the middle of the tail gate just above the license plate area. I took it to the dealer to show them the problem and ask about getting it fixed. They said they've had a few others in for this problem and that they think the plastic piece was not isolated correctly when it was attached to the tailgate. They said that the plastic expands and contracts during temp changes and it slowly wears off the paint and then rust starts to form. It's been about a month since the dealer contacted Mazda to see if this will be repaired as part of the perforation warranty (5yr/60k) since I only have 37000 miles on it. This has been a great car so far and the dealer has been really helpful, so I hope I'll hear something back soon. I guess if they won't cover it I'll just take it to the body shop and pay for it myself...:'( No biggie, but wanted to let everyone else know to keep an eye out for this before it gets too bad...
dealer denied fixing our cx-9 with this issue. Said it's not a paint perforation issue. Only covered under the 3/36 warranty which we are out of.