Rust at trailing arm mount / top well area

[otter]

Hi y'all,

I am finally getting to some much-needed work on my Spider, after a long period of neglect due to having a second project car (which I haven't done that much on either), a recently-purchased 911 SC Targa that I can't stop driving (I'd rather drive it than work on other cars) and some other priorities. So I am back on this forum for the first time in a long time with a rust-related question. I do have a more big-picture rust question ("is all this worth fixing?") but I will save that for another post.

My question here is about one specific rust area on my Spider. I have rust-through on the floor-pan surrounding the mounting point for the RHS trailing arm, at the right rear corner of the interior rear floor panel. What I want to know is this:

-is there likely to be any hidden rust in non-exterior sheetmetal inside the unibody?

The reason I consider that there might be is the known history of the car that suggests how the floorpan rust might have happened. I have had this car since about 1990, and in all the years it spent at my family's house in Atlanta, it was parked outdoors under a cover, on a slight side slope with the RHS high. The RHS top well had a habit of collecting water, likely due to a blocked drain tube. We would regularly have to siphon out water that had collected in the plastic well panel. This occurred only on the RHS. It seems likely to me that the rust-through in the floor of the car may have originated with this problem. I won't know until I can remove the top well and nearby interior trim, which I will hopefully be able to do next weekend, when it will be tolerably warm. This rust-through could have happened as a side effect of the water collecting and draining into the body, or it could have happened through more conventional means.

The car has rust in various places on exterior sheetmetal, but only two areas that need attention for safety - this area, and an area next to the trans crossmember mount. Everything else I can live with for years if I want to, except maybe the hole in the tulip panel. While repairing this area by the trailing arm would be pretty straightforward if what I see is all I have to deal with, it could be much more involved if there is interior rust repair needed, and the potential cost might change my cost-benefit calculus for the whole car.

I've attached a couple of photos below. They are the best I could manage with the car on the ground in the garage. I appreciate any feedback that those of you who have burrowed into this area may have.

Edit: how the hell do I post a vieable image link?!? I have this damn problem in one form or another on every forum I try to post images on. Argh!!

David
'81 Spider Turbo
Chicago, IL

[kmoses]

There can be a "lot" of rust hidden by the back seat support/crossmember! Under my car the rubberized undercoating looked great, when I started removing the tar sound insulation, I found the floor of the car was rusting from the inside out. While cutting out the floor using a sawzall chunks of rust started coming from under the rear crossmember. This is what mine looked like when the crossmember was removed.

[otter]

Thanks for that photo. Is the crossmember you're referring to the shelf in the interior of the tub that provides the structure for the rear "seat" ?

[kmoses]

That's the one. The crossmember adds lots strength and rigidity to the floor and the area where the lower trailing arms bolt up because it is attached to the inner rockers, floor and driveshaft tunnel. If you remove the tar like material in front and behind the crossmember, you will see it is attached with about 100 spot welds ( not really that many but a lot).

[otter]

Is the rust seen in your photo something that requires removing that panel to detect? It looks to me like that holey surface is one that is visible from below.

[kmoses]

Go under your car and scrape the rubberized undercoating off under this area. Then wipe it clean with mineral spirits, if you see any brown spots, take a small screwdriver and try to push the screwdriver into it. If the screwdriver goes through the metal, it needs to be replaced.
There were no holes visible under my car, only when I saw chunks of rust while cutting my floor did I scrape off the undercoating and saw the brown spots. When i pressed on it with a screwdriver it went through the metal like it was paper.

[otter]

Ah, I see what you mean now - thanks! I'll try that.

[pwilliam]

Experiencing the same rust as shown above. Is it dangerous to remove that cross member under the rear seat? Will the body flex or is the car stable enough that this can be removed without bracing that area?

[kmoses]

Not dangerous but it is a pain in the a**. I had my car supported at the jack points on jackstands that had plates that went around the pinchweld and engauged the flat section of each jack point. When I checked the refrence points to the ground, I was within 1/8".

[pwilliam]

Thanks for the info. What was your strategy for locating the spot welds or are they obvious once paint is sanded off?

[kmoses]

You are looking for a slight depression in the metal, some are easy to see some not so easy. What worked well was using a small flashlight. Lay the light beside the joint so the beam shines across the top of the seam and you can see the dips in the metal where the spot welds are and mark them with a marker. This is like laying a flashlight on a wall in a dark room to see where the drywall screws are located and attach to the studs.

[pwilliam]

That's awesome! Thanks for the tip. I'll document the repair. Lots of work ahead!