I live in the Midwest, so rust is always an issue with our cars. My brown 1999 sits outside 24/7 so it also has its issues these days. I bought the car when it was 3 years old and had it undercoated right away. That paid off as most of the underside of the car is in excellent condition.
However, I noticed the areas directly behind the front tires are rusted away, and there is no evidence of the undercoating there anymore. I believe it is the road "spray" from the tires that has "sandblasted" the tire-wide paths backward along the undercarriage, first removing the undercoating, then the paint, leaving rust to remove the metal over time. I used to think it was rain running down the doors, but the rust pattern underneath doesn't match that theory.
It is an expensive shame as I also bought mud flaps originally, but never installed them because I intended to protect the doors from spray, but realized they had plastic moldings along their bottoms. I guess I never gave undercarriage spray any thought.
I posted this in case it might help someone. I know mud flaps aren't trendy now, but they may save much car damage if you find yourself with a rust-free Grand Prix.
