I have several spots on my 84 123. Will see how extensive once seal is removed.

Normally how is this repaired? Cutting a good section from a donor car or welding on existing car - Brazing?

Just looking for solutions...

It all depends on how extensive the rust is as you noted. Minor rust can be wire brushed and treated. Pitting can be brushed, sanded, filled (POR15 putty, JB Weld, etc. NOT BONDO-ED!!!) and treated. Complete rust through is a more extensive problem which can lead to panel replacement.

Chad 300DT has done some extensive rust repairs using JB Weld which might be the thread you should check out.

Wait until you can see what the damage is - from what I can make out W123s are not as susceptible to this as the W126 - post up some pictures if you need help on what to do next.

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1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone

Don't leave that there - I'll take it to bits!

Rust can weaken car's structural integrity. The main cause of rust is the water which sweeps through the windows. So you should check the sealing in the doors and windows regularly and replace it as soon as possible if found damaged.

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