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Underbody Rust

K

kackle123

New member
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.
 


behind the front tires is the fender which curves under and meets the rocker, the inside fender fills up with debri from the cowl above and stays nice and moist thus your rust
 
I should have been more clear: the significant rust goes back FEET from the tire - it's not just near the front fenders. And toward the center of the undercarriage, the undercoating remains undisturbed. That's why I suspect "tire spray".

On the former show "Top Gear", I recently saw them drag-racing cars on a deserted, sandy/dusty airport runway. Afterward, they saw that the expensive automobiles had suffered tire spray damage (essentially sandblasting) during the short time they filmed.
 
Rubberized undercoating is a sacrificial layer, put there to help stones bounce off without chipping the paint finish. Normal wear and tear destroys undercoat over time, in areas where road debris impacts the body. Proper maintainence includes re-application in worn areas periodically. If you have undercoat on your car it should be touched up every year or two, in the fall season. Handy aerosol cans are available for this.

The cars have an inherent weakness in the factory-applied corrosion protection between the walls of the inner and outer rocker panel structure, so many times rust starts on the inside of the sheetmetal where it can't be seen until too late. As ITHurtz mentioned, they also have corrosion hotspots where road debris can collect... several places down low on the car. It is very important that these areas be cleared so moisture cannot accumulate.
 
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