Here's what lead me to investigate.

<img src="http://www.peachparts.com/shopforum/attachment.php?attachmentid=158701&stc=1&d=1595628157">

I examined the inside by pulling the hard cover on the right side of the trunk and found this.

<img src="http://www.peachparts.com/shopforum/attachment.php?attachmentid=158702&stc=1&d=1595628157">

I have the pieces to replace these sections from my other car and am relearning to weld after 45 years without a torch in my hand. Doing fine but still have a way to go since the mig I afforded doesn't use gas and the minimum wire is .03 which is a challenge to weld such thin sheet metal with.

You can notice the two pieces which are rusty lap as the skin turns back toward the center of the auto and the panel laps over it. As I cut the two pieces out of the parts car I noticed a type of putty between the two in that lap joint.

I used a spot welding tool so I saw the seam sealer was used originally. I've looked over several seam sealers but none of them mention being compatible with spot welding.

Does anyone know what MB used, the technique or method they used to get that sealer in the lap seam? How about it, folks. Any other old timers out there who might direct my path?

Attached Thumbnails

   

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84 300SD
85 380SE
83 528e
95 318ic

I use a weld through undercoating it's very spendy but necessary.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Is undercoating is used to seal seams?

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84 300SD
85 380SE
83 528e
95 318ic

Does anyone in or around G'ville, SC have an english wheel or sheet metal stretcher? I will pay you to run a 3/4" lip though an english wheel or pop it a few times with a stretcher.

If there are no takers on this I'm buying an english wheel.

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84 300SD
85 380SE
83 528e
95 318ic

After cutting the skin and portions of the inner support I found a half inch of rust on the inner wheel well lip.

I cleaned the dirt, seam sealer and original primer off the area then cut and butted a replacement lip.

Thought I'd post a few pigs since I almost never contribute to the forum and have enjoyed reading you folks' post, that's not to mention the wealth of information I've harvested from this forum.

Picture of the replacement part mounted with one skip weld and a clamp.

<img src="http://www.peachparts.com/shopforum/attachment.php?attachmentid=162943&stc=1&d=1628282336">

Here's the finished weld.

<img src="http://www.peachparts.com/shopforum/attachment.php?attachmentid=162944&stc=1&d=1628282429">

If you're a welder you might scoff at this but for someone who never touched a mig machine in his life I'm pretty please with the results. I didn't look at the backside to check penetration so I'll check that tomorrow and skip weld it if the penetration is insufficient.

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84 300SD
85 380SE
83 528e
95 318ic

Never forget the metal worker's mantra, "What the welder giveth, the grinder taketh away."

Give it a quick spray of "el cheapo" primer to locate any dips or bumps. The cheap stuff will come off with a splash of lacquer remover on a rag.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Yeah, I intend to use a guide coat after I work the welds down with a die grinder and coarse paper disc. I might spring for a body file. I've seen those used and it becomes a painter to find and straighten out the smallest dents.

Welding is a little like sheetrock work. It doesn't matter how good you are with the tools it's the sandpaper that makes the work right.

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84 300SD
85 380SE
83 528e
95 318ic

How is the project coming? Also, how do you treat the backside of the welds in places you can't reach?

Another thing that crossed my mind, is it possible the car was previously in a collision, and the rust you encountered was the result of the repair people not treating the repairs properly (rust with seal sealer vs etching/coating, priming, then seam sealing? Who knows, maybe MB did do it like that years ago.

Fortunately I was able to reach the backside of everything though the welding on that piece of wheel well lip was a hit or miss. I had to set the gas tip of the torch in position and pull the trigger. I got to see the welds only after removing the torch and positioning a mirror. I brushed them off will a wire wheel and covered them with the transtar 2K epoxy primer.

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84 300SD
85 380SE
83 528e
95 318ic

Sounds good, I hope it's working out to your satisfaction. There is a place, Bud's Benz in Douglasville, GA that would be an ideal operation if you choose to take it somewhere. Some things you could even mail to them for repair if you're going to reinstall yourself.