I found some rust bubbles at the leading edge of the passenger side of my W123 coupe. I'm trying to figure out how water gets behind the panels to rust from behind. Is this coming from under the wheel well? Or, is it coming down from up top and behind the body panels. If I could figure out where it comes from, I could spray some rust inhibitor back there form time to time.

I'd say that this rust is very unlikely coming from moisture inside the car. The inner wheel arch and outer panel are pinch welded and you can feel the end of the seam. Its where the paint stops and the rubberized undercoating begins.



This rust is pretty minor and I'd tackle it immediately. My guess is that there's moisture being flung from the wheels and finding any weakness in the paint itself. While there are half a dozen body plugs in both inner walls of the wheel well panels, I wouldn't disturb them as you'll cause more harm than good.



If you really wanted to go overboard on it you could pull the trunk liner on either side and reach the inside of most of the wheel arch with cavity wax.

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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]

Lower the rear window and look for access in the glass channel. Try to look with a borescope. As mentioned the trunk liner is another good way. I would remove a body plug if there is no other way.

Use something that creeps into crevices e.g., Eureka FluidFilm aerosol with an extension wand.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I'd check the rear windshield too. You have to pull the covers off the sides of your trunk and you'll get a good look at those areas.


Your hope should be that gravel has knocked a bit of paint off the bottom lip of the outer skin in that area and the rust is only on the surface. If it's coming from the sunroof or rear windshield there's a whole lot more damage than POR 15 can handle.


I cut rusty portions from both sides of the wheel wells on my 85 Sedan. I thought the bumper cover had rubbed the paint thin and it actually looked a lot like the condition in your photographs though it was all at the rear of the wheel well.


Do yourself a favor and dig into the trunk area and investigate to eliminate that issue before you go stripping paint.

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

What is the best way to remove the surface rust without rubbing the paint off?

Same way surgeons remove a cancerous growth without cutting a patient. They don't.


The rust is forming from the metal underneath the paint. You can't get rid of the rust without damaging the paint.

__________________
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]

The only way to get rid of the rust is to cut all of it out. That's why it's imperative to start immediately and don't stop removing paint until you've uncovered all the rust then don't stop cutting until it's all gone.

Por 15 is a good product for surface rust but if the rust came from the backside of the panel it's not surface rust.

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

I'm a believer in seeking out a second opinion also known as a second set of eyeballs on the problem. Check out both google and yelp.com reviews for a competent body shop (4.5 stars or better). Make an appointment to have your car carefully inspected for:

1. Surface Rust

2. Deep rust

If number one, you may be able to deal with it easier, if number two, then major surgery may be required. Still, a competent body shop can assist you with making the right decision.