Just did some restorative paint and body work to put a halt to some rust on my 02 OBW. On one side of the car, the rear wheel arch had some surface rust starting to creep up from inside the wheel well to the surface paint, right at the plastic body cladding top seam on the dogleg and again at the rear bumper cover top seam. The culprit appears to be the rubber seam cover that the factory pushed onto the seam where the floorpan and the exterior body are welded together. If you follow the top of the rear bumper cover to where it ends at the rear wheel arch, you'll see this rubber cover as it bends around the top of the wheel opening, ending at the top of the rear dogleg lower cladding. It is simply pushed on at the factory. The rubber seam cover holds salt and water, and on my car started the rusting at each end of it. Check your OB for rust signs in this area. After my repaint I filled the cover with waterproof grease and pushed it back onto the seam, after having greased the daylights out of the interior wheel well edges first.

Good advice.

This is a common issue on some hondas and probably other makes as well. Definately pull that rubber cover off and fill it with undercoating grease or something then slam it back on. Some people even just pull it off and leave it off but I personally think the grease option is better.

My Impreza started rusting this exact same way.

 

When I got my Legacy, those rubber strips came off and stayed off.