Only had my XK8 about 6 weeks but think i have something to share. The source of the rust on mine has been a leaking back window ( back to 2006) and the roof rail. Check for a gap between the roof rail and the roof and fill it with black silicon right down to the bottom of the front window. Check the rubber at this point is directing water away under the wing not down onto the carpet, Check the scuttle drains are in place ..in the boxes each side of the engine compartment.
Pull up the back seat .My upholstery was sodden and the RH full of water. When you brake this slops forward , goes under the carpet and down to below the pedals where it rots the floor. I took all the carpet out and removed the foam from the rear .The source is the bottom corner of the rear window. Below the corner is a plate . If the car is parked nose up water runs backwards into the trunk and wets the carpet .If nose down it goes into the back seat and into the sill hence the rot. Push a rag under the rear corner of the screen from in the boot and check if it wet after its rained /hose the thing.
Once you have the leaks sorted ( Silicone) you can wax the sills inside. I used what in the UK we call Waxoyl 5 litre tin about £25 and an air operated Shultz gun (ebay) .I replaced the cartridges with a UK 1 litre plastic milk bottle pushing the cap into the original fixing and securing with a worm drive clip around the central pipe.No air hole the bottle collapses as you go . I think a round quart tin will fit.
There are three access holes to the sill all with 16mm rubber bungs. On the rear one central to the sill section, Wheel off .bung out . Probe the hole and you will realize you need to spray to L and R to avoid obstructions. At the front behind the lower wheel arch liner are two bungs ,one central and one in the top outer corner. Blast away. The wax will travel at least 6 feet from the gun.
Then for the rusty sill return. Scrape and clean and dry with a heat gun and then coat with Marine Epoxy. Not the more common Polyester as it sets too fast . The marine epoxy takes 24 hrs in which time it is drawn into the rust and encapsulates it . TIP for an epoxy brush use a 1in blob of foam held in some long nose pliers so you can throw it way. Re coat as the left over epoxy starts to go off.
Hope that helps and is not a repeat of other posts.
If this is useful put a comment or perhaps I wont post on similar subjects again