Rust ? Ohhh Yhaaa!!
Well, when it got down to 99 today (Aug 8th) , I went out to start seeing what was going on with the XJ12. I've addressed the rotten floor pan on the drivers side in the past, but only (mostly) as just visually accessing the rust. Well this afternoon I went "hand on" and found that the FULL floor pan is rotted, BUT the rot was pretty much isolated to just the floor pan, but pretty, like the one board that has termites and every thing around it is much less affected, AND very fixable.....by ME.
In the anchoring places around the floor pan, where the floor pan connect to the tunnel on the right (while rusted) was still solid, and in very recoverable condition. On the left side, (the door sill side) there was only about 40-60 left. The upper 50% of the door sill was very solid and there's enough good metal left so that I can rivit, or glue or weld new metal to it and make it solid again, attack NEW metal to it. BUT The whole floor pan is gone, BUT .
YES! Rust can be an issue on older Jag's.

Rusted out Drivers side floor pan
When I made my (accepted) offer on the XJ12 I WAS aware that it wasn't running, and hadn't been running for some time.... Little did know that the "some time" turned out to be 21 years!
So... NBD. With a 50+ year old car when I bought it , I knew that there would probably be areas of rust. I don't think that the gent that I bought the car from had any idea that there was much rust on the car.
While this may look like a big job, it really isn't. A new floor pan is only (around) $135.00 and not a big deal to cut out and weld in. OR, I can use the new way is to GLUE it in.
I think that I'm going to weld it in.

Rusty inside Rocker on Divers side
I'm pretty sure that to REALLY check out the exten of the rust on the floor and rocker sides I need to remove all of the carpeting. Which I haven't done yet.
After a general survay of the car, one of my FIRST task is to simply (after 20 yars of not being fired up) of not being started is to just turn over the engine, and check the statuns of the electrical system.