I restored a W116 over a few years. It was very rusty underneath and took plenty of welding - new rockers, floor pan sections, wheel arches etc. It was really amazing how sections that looked completely sound turned out to be rotten. You have been warned.

The best thing you can do to prevent rust is have the underside waxed, and inject all the holes MB thoughtfully provided with wax too. It costs very little compared to the delayed cost of the damage a few years down the line.

I used DINITROL and a compressor powered gun, but I'm sure you could find a good garage to do this for you.

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'79 280SE
'87 560SEL
'83 280CE
'01 Nissan Micra
'98 VW Passat
'83 911 turbo