multi-layered sheet metal

[Robert Bondi]

I'm working on the restoration car addressing all the rust spots with the rear end stuff all removed. After getting into the worst spot on the car, I'm still a bit surprised at how this is constructed. On some of the worst spots on the car, the good news is there's actually multiple layers of metal with only one of them destroyed; on the other hand, it's also this multiple layer design that provides an opportunity for water to get trapped and start rusting. Is this common for body work or just a BMW thing of this time period?

There are at least 3 layers of thin sheet metal in the region immediately in front of the right rear subframe mount enumerated
in the pictures:

layer1: Ths is the outermost layer, totally destroyed and mushroomed out around the corrosion epicenter.
layer2: Ths layer has a heavy rust, pitted outer surface, but is intact. It also has a perfectly machined hole in it, clearly there by design. Is it a weep passage between layers? There may be multiple layers in this middle region.
layer3: In the picture illuminated by flash, we clearly see into the cavity to layer 3. It looks like a factory primed virgin surface. I'm fairly certain this is backside of the sheet metal surface of the interior. I was doing floor work from the inside last year and noticed no serious issues through the floor here, which now makes sense because of the multiple layers. I'm wondering if the open volume connects to the weep holes that run under the doors on both sides....I blew compressed air in that perfect hole in layer 2, but did not detect an obvious air flow out the nearest weep holes.

There is open volume between each of these layers, which also makes repair tricky no matter the method chosen. I'm still going POR15 as with most everything on the underside. New interior is also behind that inner layer of sheet metal now. This inspection also indicates all this sheet metal is fairly cosmetic, so clearly the serious steel taking the load from the subframe must be elsewhere.

Other question is what to do with that designed hole. I'm now leaning to preserving it but sticking a grommet there with a smaller hole in case there is a weep function. Definitely open to suggestions if somebody knows what that is.

Damage area without flash:



Damage area with flash:



Clean up in progress:

[dutch]

I`m a couple days away from dropping my rear subframe, so I will have a closer look at the construction of that area. From what I could see on mine after clearing some ubc I could reach, there is no 3rd layer. Mine is a `76 series 1 528, so there might have been an upgrade in that section..? Or maybe I just don`t see it yet. Anyways...

As far as stupid designs go, There was no intension to make these cars last for more than 10 years or so. By then it would be time for the customer to go get a brand new e28. Also sheetmetal quality and rust prevention were nothing like today...

I do think you need to rethink your phrase ; " This inspection also indicates all this sheet metal is fairly cosmetic, so clearly the serious steel taking the load from the subframe must be elsewhere."

There is the floorpan with the 2 or 3mm bracket on top which holds the rod that the subframe bolts to. That bracket and the rod get strength and stability from the box it makes with the floorpan and the "third layer"
That layer is gone and the floorpan , which is 1mm (19GA) is pitted so deep ,it has probably less than half that thickness left.
Imho, there`s nothing cosmetic about that area and you need more than por15 to fix that?

As for the wheep hole; I would punch a smaller hole in the repairpanel and plug it after rust treatment.

I`ll let you know what I find when I drop my frame.

[Robert Bondi]

I hear you. Great response and info! It sounds like you have much more experience with body work than I do, which is near none.
I've been trying to figure out what I'm looking at in that area and have gone back and forth in my mind about how bad it is. It sounds like I've underestimated how much strength is provided by even thin layers of sheet metal forming a complete hollow cavity. I've had additional conversations offline that suggest the same.

I should look at options to do a welding repair in this spot.

Major concerns, as my extent of welding experience is just a few exhaust tack welds ~15 years ago:
1. thin, curved sheet metal is not easy, especially for the novice
2. welding under the car is another challenge for the novice
3. I'm concerned about transferring too much heat to layer 3. I've got brand new carpet and seats on the
other side of layer 3, the interior side floor pan. I don't have a feel for safe distance on this aspect (I'm likely using MIG)?
4. I've also got a tank of fuel sitting nearby since this is the right rear. That's supposed to be a separate project.
Perhaps that can be shielded with a welding blanket.

It sounds like you are working some similar projects. Let me know what you see on your car in this spot when you get a better look.

Thanks,
Robert

I`m a couple days away from dropping my rear subframe, so I will have a closer look at the construction of that area. From what I could see on mine after clearing some ubc I could reach, there is no 3rd layer. Mine is a `76 series 1 528, so there might have been an upgrade in that section..? Or maybe I just don`t see it yet. Anyways...

As far as stupid designs go, There was no intension to make these cars last for more than 10 years or so. By then it would be time for the customer to go get a brand new e28. Also sheetmetal quality and rust prevention were nothing like today...

I do think you need to rethink your phrase ; " This inspection also indicates all this sheet metal is fairly cosmetic, so clearly the serious steel taking the load from the subframe must be elsewhere."

There is the floorpan with the 2 or 3mm bracket on top which holds the rod that the subframe bolts to. That bracket and the rod get strength and stability from the box it makes with the floorpan and the "third layer"
That layer is gone and the floorpan , which is 1mm (19GA) is pitted so deep ,it has probably less than half that thickness left.
Imho, there`s nothing cosmetic about that area and you need more than por15 to fix that?

As for the wheep hole; I would punch a smaller hole in the repairpanel and plug it after rust treatment.

I`ll let you know what I find when I drop my frame.

[dutch]

Will do. Plan is somewhere next week but in reality, my plans don`t always work...

My main concern would be keeping that rod in position. It all depends on what quality level of repair you`re after. If you want to patch the pitted panel, the whole bunch needs to come out, probably including the bracket that holds the rod.

Just start wire-wheeling everything as clean as you can wide around the rusty area to make sure you can actually see what you`re dealing with.
That 3rd layer would be spotwelded to the floorpan I guess ,so make sure you get the entire panel exposed. Please post some pics of your findings. I will try to help you from a distance, but it looks like you might need some experienced help along the way...

As for your concerns;
- The welding experience... should be good to go. ...probably not...
- making a thin curved patch panel is much easier than a heavy curved panel.
- welding under the car is the same as from above. You just need earplugs.
- You need heat or the panel will fall off. The welding will be done under the rear seat so that has to come out ( 2 bolts?) Make sure you wiring is out of the way! I would opt for a bunch of wet towels and eyes inside the car while welding. Mig would be the way to go.
- I would not be too worried about the tank, unless it leaks. Make sure no sparks can fly on top and you should be fine.

Work on one side and keep the other side in tact for reference.

[dutch]

Ok, I dropped the carrier and here`s what I think is going on;

- Layer # 1 in your pics is the floorpan. When wire brushing you will find a foam gasket between the floorpan and the guide bushing for the threaded rod. That said, the floorpan does not add any structural strength to the mounting point.

- Layer # 2 is the lower part of a 2 part bracket you can see when you pull the rear seat.

- Layer # 3 is the top part of that bracket. This whole situation is welded to the floorpan, so your main problem here is that the floorpan is rotted right up to where the bracket is welded to it, which makes it structural.

It`s kinda hard to see from the pics how bad the pitting is on the bracket actually is and I don`t know what quality level you`re after. That part is 2mm ( 12 ga) so unless you plan on driving ralley`s , you might be ok with a serious clean up, rust converter and some kind of coating after... I would start by cutting that floorpan until you`re in healthy metal. There`s no need to worry about the position of that rod as the bracket is welded in the corner inside the car.
That section of floorpan is not that hard to make for someone with some understanding of sheet metal shaping.
Hope this helps.

[Robert Bondi]

Excellent! Thanks for the assessment. I guess it's serendipitous that you happen to be working on similar regions of your car; I'm not sure how else I would have landed this specific information. I had contacted a local body shop and shared the same pics....had a conversation, but didn't hear back on it.

I've mostly been working on all the other minor rust restoration regions since your last input. No, I don't drive my cars very hard....no rallies, racing, nor auto-cross drills. My plan of attack is to repair that outer floorplan (1) with a welded repair panel after cleaning the area up as best as possible. I don't want to do any welding in the inside....I see no way to not have sparks flying on everything. I will dig up my pictures of that area from last year....hopefully will give an idea of the other side. It seems feasible to remove the back seat bottom and maybe pull up a little carpet as needed for some wet towel on the inner side to avoid heat transfer from the welding on the exterior. IIRC, the head of that subframe bolt on the inside is under the rear seat, which actually means no immediate carpet covering to worry about.

Yeah, I did notice that foam gasket around the guide bushing.

I just dropped some serious coin on a nice Lincoln MIG welder (215MPi) and my welding PPE also just arrived today. There are a few other minor repairs that really require welding, so hopefully a good investment in the long run to the tool collection.

Thanks again! I'll drop some more pics on this thread as I move forward.

Robert

Ok, I dropped the carrier and here`s what I think is going on;

- Layer # 1 in your pics is the floorpan. When wire brushing you will find a foam gasket between the floorpan and the guide bushing for the threaded rod. That said, the floorpan does not add any structural strength to the mounting point.

- Layer # 2 is the lower part of a 2 part bracket you can see when you pull the rear seat.

- Layer # 3 is the top part of that bracket. This whole situation is welded to the floorpan, so your main problem here is that the floorpan is rotted right up to where the bracket is welded to it, which makes it structural.

It`s kinda hard to see from the pics how bad the pitting is on the bracket actually is and I don`t know what quality level you`re after. That part is 2mm ( 12 ga) so unless you plan on driving ralley`s , you might be ok with a serious clean up, rust converter and some kind of coating after... I would start by cutting that floorpan until you`re in healthy metal. There`s no need to worry about the position of that rod as the bracket is welded in the corner inside the car.
That section of floorpan is not that hard to make for someone with some understanding of sheet metal shaping.
Hope this helps.

[dutch]

The lincoln is a nice investment. Are you going to use gas or flux core wire? ( I would definately use gas...) Just get a bunch of 19ga sheet and go practice practice practice. Different angles ,gaps and welder adjustments etc.

The repair on your floor will not go with no sparks inside... go get some welding blankets ,have extra eyes and a fire extinguisher on hand. Better safe than sorry. Good luck. Looking forward to some pics!

[Robert Bondi]

I'm probably going flux core. Gas sounds cleaner, but then it's rent or buy a cylinder and fill it....another research project. I don't care how pretty the weld is given the location. The top goal is instead protect the interior. I'll see how my practice welds look.

Are you saying welding on the outside of the car is going to produce sparks inside? I guess I'm not quite seeing or understanding that unless there is burnthrough, especially with the multiple layers of metal with gaps between them? I Will be doing welding blankets, wet towels, fire extinguisher, and extra eyes regardless, so I am going the extra mile on all precautionary fronts as the plan.

I located some before and after shots from 2023 of my interior work focusing on that right rear subframe mount from the inside. I'm now curious about pulling the rear seat and seeing if there is an obvious path from inside to out through those machines holes and making sense of the different layers from both sides.

Before....water was getting in through rust holes in both rear wheel wells, creating those rust tracks. The ancient adhesive and old foam also looks orange/brown and gives the illusion of more rust than there is. There was still plenty of rust. I really hit the POR15 product catalog hard on that floor work.



After....cleaned up very nice and why welding on the inside is basically not on the consideration list.

[dutch]

I think you will see what I mean once you have exposed / removed all rusty metal.
The sound deadener in the last picture is exactly on the edge of your " layer #2 " The round part on top of that is #3. The rotten floorpanel is
( I think , but I might be wrong ) stretching beyond #2 underneath the sound deadener. So, once you remove that, you`re inside the car.

You could use some kind of hacksaw or air powered mini saw to cut and use a tig welder to reduce sparks. Downside for tig is your metal needs to be really really clean.

Whatever route you choose, make sure you work healthy metal. Cut it clean and then some. Just my 2 cts.

[Robert Bondi]

I pulled the rear seat and now I understand everything you've been saying. I've enumerated the same metal layers from the
inside perspective confirming your last message. I'm actually a little more optimistic about the situation now. Layer2 is somewhat rectangular and only the side nearest the camera is most cause for concern. Pictures are backlit showing the slit hole from inside to outside. The hole itself is not damage, but instead a gap between two weld seams. Good news is the rot in layer1 doesn't extend beyond the footprint of layer2 and the welds are in good shape, IMO. I like the idea of a sheet metal repair panel slightly larger than that hole welded to the floorpan from the outside.

Enumerated image with backlight, inside view. Weld seams on the damage front indicated by red arrows. Sound deadener appears to go right up to layer2 only because I flat black painted over the deadener and about 1" of plain floorplan right up to layer2.



Backlit image, outside view



Basic plan for this area:
- Dissolve rust on layer 2
- Possibly add additional weld seam between layer1 and layer2 at the slit in the picture
- Cut/Form/Prepare and MIG weld a repair panel (18 or 20gauge) slightly bigger than layer1 hole from outside
-

Throw weld blanket(s) over this region at all times even for exterior welding

[dutch]

looking forward to see how this pans out.
Personally I would cut the welds that hold # 2 to the floorpan, cut out a larger portion and do most welding from the top side, but I understand your concerns. I do sheetmetal work for a living so there`s a difference in looking at repairs like this... Please let me know when I talk poop for someone with little experience

Apart from the obious like taking care of your wiring when cutting and welding, please make sure the beads in the floorpan which run underneath the lower seat support panel are plugged solid. They are great canals for weld sparks to roll under the carpet.
Good luck.

[Robert Bondi]

Last weekend was initial welding practice. No fires, no injuries, and things started to look better with each successive attempt, so a good start. Things would likely look better if I invest in a few extra items for the shielding gas in the future. This was just 0.035 self-shielded flux core Lincoln wire. The auto darkening helmet was worth the extra dollars. I still find it difficult that one needs the hand-eye coordination of a surgeon, yet those giant leather gloves give me the dexterity of a walrus, as far a making nice pretty beads.

I still have to make the repair panel. I'm going with 20 gauge cold-rolled steel. What's the best method to cut it? Seems a little too thick for even my best aviation shears....I get about an inch in, but then the steel is too thick to really fold away to advance the shears further. I ended up cutting straight practice strips with my angle grinder and a cutoff wheel. I got a sand-filled Eastwood beater bag and tear drop mallet for shaping....zero experience, but hoping that lots of strategic trial and error gets me there.

Highlight practice pictures:

The porch practice work space.



Initial tack weld practice on some M6-1.0 weld studs. I actually broke a few of these in the drive shaft tunnel for the heat shielding, so also on my repair list. I found that a 3-point tack should work nicely. Right to left shows nice improvement.



My 1'x2' practice sheet. Lots of stuff here...weld studs, a right angle piece, practice with different bead patterns, and a 20 gauge strip welded over the 18 gauge sheet (should resemble the actual repair on the floorpan).

[dutch]

progress!
I can`t see the welds without close up pics, so no advise there...
I never use the trigger side glove. Use the other hand to support the trigger en shield the hand at the same time. This gives you way better control.
Make sure you`ve got enough light on the project. If the helmet starts acting up, adjust the senitivity on it. Just make sure you can actually see what you`re doing.

As for the cutting; you can`t cut through the middle of any thickness sheet with tin snips. First make a paper tamplet of the shape you need and add 3/4" or so all around. Cut rough shape with an angle grinder ,thén use your snips to cut the line. Not shure what kind of snips you have but I use Wiss. No problem cutting scribed lines up to 18Ga. For the panels you need now, just start with a square. See below...

Shaping the panels you need don`t requier the mallet and shot bag. You will probably end up with a bunch of dented experience...
BTW... do you need to replace both sides?

I will try to explane what I think would be the easiest way...

Apart from the cup around the threaded rod, there is not a crazy lot of shape in these panels, so for now put your focus there.
-Cut a hole in 2 pieces of plywood , the size of the widest part of the cup. The wood needs to be the same thickness as the cup`s depth.
-Cut a plenty large square / round / rectangular piece ( or 2 if you need both sides) of sheet metal and drill or cut the hole for the threaded rod.
( maybe a tad smaller since it will most likely stretch a bit. )
-Clamp sheet metal in between plywood with the large hole in the center. Drill a few holes to bolt the sandwich together. Then clamp the lot flat and solid to a workbench and slowly work the metal down with a ball peen hammer or even better a plastic / nylon punch with any hammer. Work from edge towards center.
Note; since the cup in the metal is deeper on one side, I would make a wedge to lay on the bottom of the hole in the wood so you can wor the metal to that surface.

[Mike W.]

Use a power shears to cut thru the sheet metal. I've used them before, not necessarily the brand pictured, and wow, they work well. Not that expensive either, the one pictured is something like $42 at Amazon.

And FWIW, while a royal pain with the tank and everything, if even an option, using an inert gas shield instead of flux core makes for cleaner welds.

[dutch]

In the previous post I stated the cup would be deeper on one side, but that`s not the case. I just checked cause I wasn`t sure. Took some measurements while I was there. The plywood ( or whatever material ) you hammer the cup into, has to be 5mm thick. The hole in that piece has to be 75mm and the hole for the rod is 35mm. ( inches confuse me...)
The original metal thickness is 19Ga. I would not use thinner if 19 is not available.

The rest of the panel should be bending only, no shaping.
I agree with Mike. Use gas if possible. No experience is one thing. No experience and fighting your machine is something else...