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Old 08-08-2015, 12:22 PM   #4 (permalink)
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Originally Posted by Speeed3 View Post
How do you think the rust started?
It looks like it started from inside. Rusted trough wheel well got dirt in there?
It's definitely rusting from the inside out. The rust is a lot more widespread than the photo shows because once the paint has blistered, the metal beneath it is all corroded.

I haven't opened up the rocker panel yet to confirm, but I believe the problem has to do with this "shelf" at the bottom of the rear wheel arch (ahead of the rear tire) that holds dirt, salt & moisture:

You can't see it well in the pic, but where the yellow arrow points, the tip of the screwdriver that I stabbed into the car is visible at a small hole there. Which means water, salt & dirt are getting in the rocker cavity there.

Also, all the factory undercoating has flaked off that shelf, exposing bare metal.

Maybe you (mostly meaning other Prius owners) can put rust preventing oil into there from rear spat attachment hole.
Exactly! I would highly recommend drilling the rocker panel where the rear doors close (above the door seal) to spray something for rust prevention inside the cavity.

And when I cut the existing rust out of the rocker panel to repair it later this summer or early fall, I should be able to confirm the source.
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