This is my car and what I done, basically take out all the rear interior and rear seats allowing access to the inner arch and I sprayed it all with a cavity wax.
Take off the front, rear bumpers and remove the sill covers and spray cavity wax and clear underseal everywhere. I went for clear because my car had no rust so I didn't want to just cover it in black.
Excuse my rubbish phone pics.
You can see the openings on either side to get access to the rear quarters here:
This is what the inside of the wheel arch looks like from behind the rear speaker hole:
I had a 360 degree wand that comes with Dynax S50 that I could insert in these holes and basically cover the whole of the inside of the sill in cavity wax.
Weird double skin of the wheel arch, no wonder they rust here with stuff getting trapped in between layers.
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