Unwelcome rust

Hi guys,
Discovered some unwelcome rot on the right-side trunk floor panel of my new-to-me 3.2 (uncovered when I accessed the cat ecu electronics). The first photo is from inside the trunk, looking down; the second is from under the car looking up. The good news is that the corrosion appears to be confined to this one panel (the third pic is of the corresponding piece on the left-side, taken from under the rear bumper).
Question 1: is that part available (on the schematics on the Ricambi site all the pieces are labeled "insulation" suggesting that they have only the foam material, but the diagram looks like it's referring to the metal bits).
Question 2: Any suggestions for the best way to protect the electronics in that compartment in the meantime?
Question 3: Any speculation how it could be that this panel is so rotted while the surrounding metal appears unaffected?
Many thanks!
Hugh

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Thanks for the reply. I'm not sure a welder would have enough to work with after I break off all the rot from that piece (?).
Do you -- or anyone else know -- how difficult it is to remove that piece?
Is my rusted piece in this diagram? Is it #31? (I have trouble relating the diagram to what i see in my trunk.)
https://www.ricambiamerica.com/car-diagrams/ferrari/v6-v8/mondial-group/mondial-3-2/luggage-and-passenger-compartment-insulation-3-30.html

 

Yes, it's definitely the US version; I didn't realize it was part of a larger piece. Getting a good welder seems to be the way to go.
It's somewhat reassuring to know that the original part is around $500 or so; I'm hoping I can get it repaired for less than $1K.

Thanks!

 

Quoted between 3 and 4K. Yikes. The body man said it was a pretty straightforward job, just incredibly time consuming, as fabricating replacement panels and butt-welding requires tons of patience. Might be a DIY job for yours truly.

 

Thanks for the perspective. I'm sure I could have it done more cheaply here, too, but at a certain point you wonder whether it will be done properly. That's why I'm leaning towards doing it myself.
BTW, I asked the guy what it would cost if I sourced the part, and he said that if anything it would be more expensive. Since 7/8 of that piece is unaffected, they would just cut a bit off of it anyway.

 

That's a great view -- I think I can tack weld custom cut sheet metal to the outside. Hopefully I won't have to remove the exhaust.

On a related note: do any of you guys know if there's a piece of insulation at the very bottom of the floor plan on the right, protecting the electronics from the exhaust heat? I think that would explain how that area rusted so badly -- the material got waterlogged and kept the metal underneath wet. But there was no piece of insultation there in my car and someone on another thread said that there was none in his either.

 

Removed the bumper and the tailpipes today. The good news is that the rusty area in the lower right corner is very accessible. Bad news is that there's a bit more towards the center of the car that curls around towards the floor piece that is riveted onto an (unaffected) piece beneath it. I'll post a photo tomorrow.

In the meantime, any tips on removing the trunk carpet in the 3.2? It looks like a single piece that is tucked under the rubber trunk seal. I'd like to remove it without destroying anything. Does trunk seal lift out?

 

You mean the glue attaching the rubber seal to the car, right?

I'd like to avoid simply ripping the carpet away from the seal (first photo):
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Second photo: here's the part I'm a little unsure of, specifically, the horizontal part. I'm sure I can weld from inside the trunk, but I'm worried if the rust extends too far inside, I won't be able to get epoxy primer in the narrow gap between the upper and lower panels.
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The third photo shows the tricky part. This is looking directly down into the trunk from the rear bumper. The thin sheet of pitted steel lies on top of a panel underneath which is itself unaffected by rust. Cutting a patch to fit shouldn't be a problem, but protecting the underside is, as it won't be accessible for priming. I'm considering welding a piece of thin stainless steel, But, apart from the challenge of mig-welding stainless to mild steel, my concern is that even a stainless weld will be vulnerable to rust. Any suggestions? Image Unavailable, Please Login

 

I probably made it hard to visualize by sliding a thin piece of metal in between the lower piece and the corroded piece above -- it's clearer in the first photo.
The issue is the underside of the piece to be welded in, which almost touches the piece below it.

 

I'm not familiar with a plumber's heat mat -- is it a thin piece of copper?
Yes, welding a piece isn't my concern. It's protecting the underside of the piece I weld in. Once in place some of it won't be accessible for priming. Since there's an overhang, about half of the underside surface will be accessible. It's the other half, though, along with the weld seam itself, that will be blocked by the piece underneath.

That's why I plan on cutting away the absolute minimum, leaving somewhat pitted metal. There seem to be three options:
1. use stainless steel (welding stainless to mild steel might not be easy, particularly for someone who has never welded before. And even using stainless wire, the welding process will compromise the corrosion resistant properties of the stainless).
2. apply epoxy primer on the underside of the piece I weld in before welding it (although the heat from the welding process will probably disintegrate the primer).
3. weld the seam on the back side and pry open a slight gap and inject primer between the two plates before welding everything up (I fear the gap will still be two small, however).