I just noticed that my coupe has these rust spots just behind the front wheel. Does anyone know how water gets in to this spot?

There's plenty of ways it could get there.

Leak from hinge pocket into footwell and into rocker or water getting into rocker area through self tapping screw penetration.

It could also be surface corrosion from the jack penetrating the undercoating.

Either way its right around a body plug/grommet and the seam between the rocker panel and floor. That area is going to trap any water that gets in.

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Onus probandi incumbit ei qui dicit, non ei qui negat

I recondition w123/w126/w124/w140/r107/r129/ steering boxes!


1984 300D "Elsa" odo reset 6/2011 147k
1983 300TD "Mitzi" ~268k OM603 powered
1995 E300 "Adelheid" 262k [Sold]

Any ideas on how to prevent it? And clean what has happened so far?

Well, finding out where the water is coming from would be where I would start. I'd clean out my hinge pockets of any leaves and debris, inspect the inner wheel well (remove the plastic inner fender) and check the footwell for any other signs of water. Visual inspection of everything, then maybe pouring some water in those areas to make sure its draining properly. Check the windshield and sunroof while you're at it (even if they aren't the likely sources of this rust).

Wire wheel back the undercoating to expose the metal, you'll find out whether its simply surface corrosion or if there's been metal loss. Cut out any rust back to solid metal, weld in new metal (primed with weld thru primer), paint, remove old body grommets nearby, spray cavity wax in the rocker panel, replace body grommets and replace undercoating in that area.

It is a bit of work, but it won't ever happen again if you're thorough.

__________________
Onus probandi incumbit ei qui dicit, non ei qui negat

I recondition w123/w126/w124/w140/r107/r129/ steering boxes!


1984 300D "Elsa" odo reset 6/2011 147k
1983 300TD "Mitzi" ~268k OM603 powered
1995 E300 "Adelheid" 262k [Sold]

Mercedes coating I consider a noise reduction coating. More than a rustproofing one anyways. I cannot remember but it is probably not inside the rocker panels.

Drilling the typical 1/2 inch holes for the plastic caps and spraying in oil and grease mixed. It will creep everywhere in there and give decent protection. Attempts to rustproof Mercedes cars of that vintage overall are almost impossible. Driving them here all year long is suicidal for these cars.

Volkswagon used a great system from the factory. Production levels did not allow the Mexican workers to use it properly. It was elecroplating a thick layer of sacrificial metals on the shell. It was hit and miss if you got one done properly as intended by Volkswagen.

It added substantial weight to the car as well. That is why they had the twelve year rust penetration warranty. Yet it pretty much took a court case to get them to honor it. As they are all too aware that the quality control was lacking.

On the Jetta the wife totally wrote off there were two areas of separation from the parent steel. At least 1/16 inch thick. Of sacrificial metal. Those shells had to stay in the electroplating tanks for some time to get a coating that thick built up. I believe that program no longer exists as it prevents rapid production. I know their 12 year warranty is gone for a long time now. Against rust perforation. Wolfsburg designated Volkswagens where made in Germany. My feeling that quality control there was kept up.

To be fair to Mercedes their market was primarily people that do not keep the car too long. They would have been all too well aware of what their coating could result in. Being in the proverbial rust belt or in certain climates.

Volkswagen does not do well against owners that use small claims court. Volkswagen customer service reps where given a list of 10 or 12 denial of rust claim items.