I have almost finished the rust repairs on my 240D, but am struggling with the last rusty area. The problem is along the passenger side rocker panel on the inside of the car. In this area there is a 1.5 inch wide lap weld where the floor bends upwards to form the inside wall of the rocker panel (which is running along the top of the photo) and overlaps the bottom of the outer wall of the rocker panel. A leak to the interior of the car allowed water to get between the two panels and cause the problems you see here.

I am not sure what the best way to repair this area is. After patching the holes with no metal behind them my two ideas are to 1) and try to flow some sort of rust preventer between the two layers, or 2) cut and weld along the seams (red lines in the second picture) and make the lap weld into a butt weld by removing one of layers of metal.

Any ideas or suggestions would be appreciated.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

In order to retain the structural integrity of the uni-body construction, you need to remove that rusty metal and weld in a new piece. Rust inhibitor just does as it's named, it INHIBITS rust! It won't prevent it from returning with a vengance or worst of all just encapsulates it waiting for a chance to oxygen to return. Sad experience taught me that!

What he said. All the bad gotta come out, and weld new in.

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95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K

So what do you think about making this lap weld into a butt weld? I don't think I will loose much structural integrity by eliminating the 1/32" thick or so floor metal layer. The underlying outer rocker panel layer is around 1/16" thick.

If I do replace the rusty floor metal in this area and recreate the lap weld what could I use in the lap area to stall rust from reforming in the future?

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

That's a tough one. Clean the lap up the best you can and seam seal it with some good 3M or Transstar seam sealer. If rust has started in those crevices, there isn't a whole lot you can do other than keeping moisture away from it and hope.

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95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K

Any experience with this stuff?

http://www.levineautoparts.com/upwelprimaer.html

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

We use a fair amount of U-Pol products. VERY good stuff,,,, you won't go wrong with anything with that name on it.

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95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K

It all depends on what your goal is. If you want to just patch it up, then there is any number of approaches you can take. If OTOH you want to fix it for the long term, cut out ALL the nastiness and form a new piece using metal cut from an old hood or fender. Take whatever time is necessary to cut and bend a piece that fits the cutout with a 1/16" gap. Grind everything clean and coat with weld through primer. Clamp it in place and then weld it in with a series of small MIG welds, not much more than a tack. Don't pool these tacks. Skip around all over with these tacks and take your time. This is so that you don't allow warping heat to build. When it is all welded in, grind off most of the weld, but not all of it. Coat all welded areas with fiberglass reinforced body filler and then grind that off leaving enough in place to seal the welds.

This method, if done properly, will give you a forever repair, or at least as forever as it can be depending upon road salt and the like.