I have mentioned this type of damage before when responding to people doing fuel tank repairs/replacement
If your rear screen is leaking at the lower corners (rear parcel shelf is always wet) and you ignore it, This is the result and most likely the reason you will have to scrap the car, eventually..

This rust comes from water leaking past the screen seals and running down the inside of the wing and eventually rusting through - as in these pictures.

So before poking big money into the car on various other items, I would pay particular attention to this area before anything else. Remove the fuel tank and take a look. If rust is beginning to form ,..repair the leaks and take care of the rust.

These pictures are from Craigslist Seattle / Tacoma area. Note There is also a nice white low mileage XJ6 listed there as well I am not affiliated ,,,just watch CL for things I may need, and thought I would pass along the info to anyone who is an enthusiast and looking for a good non abused low mile Series 3 Xj6

 

Alyn: you are right about this. The water is the cause but the reason it happens in a vertical line is that there is foam packing right there and the foam absorbs and retains the water.

 

Yes are right...there is an interior steel panel that runs from side to side under the rear parcel shelf and has the foam attached to the ends. And yes it does retain water that has reached it by leaking past the screen corners.

In fact if you have leaking rear screens the water also dribbles down into the cabin interior and comes to rest under the rear seat squab,..puddling there on either side of the driveshaft tunnel. That's where the moldy dampness comes from. If the water gets deep enough under the rear seat, the carpets get soaked and then the water is leached from the carpets into the interior door card - ruining it. Floors start rusting.

I have posted here about this before and usually when the fuel tanks are out ,..is the best time to have a look up into the cavity and see if rust is forming. I sprayed POR 15 up on the inside of the panels and then sprayed the exterior of the fuel tanks as well. This stuff is GOOD.

Not many people understand this as one of the areas to be examined , whether buying the car, or contemplating a bare metal respray, A shame to have the car repainted only to have rust from the inside of the rear wing finally eat through the the panel and become evident as the new paint bubbles