Hi folks,
My '85 300D has started to get acne in the form of a couple small rust bubbles forming under the paint on the rear fender (on the back side of the wheel well), and I'm heartbroken. The car's been repainted at some point in the past, and I think the rear fenders were replaced (spot welds inconsistent). I know that the only way to fix this properly is to replace the fenders, or at least cut out ALL of the rust and replace it with welded in patch panels. I also know that I've not got the money to pay someone to do that work, plus paint it. I'd eventually like to buy a mig or tig welder and learn to do the bodywork myself, then potentially pay to have the fender blended back in, but that's not going to happen for a while. I'm assuming the water's leaked in around the rear window seal, or possibly around the fuel filler cap.

Is there any way to get some rust inhibitor in between the inner and outer fender panels without major surgery? I know it's not a long term solution, but putting some oil based rust inhibitor in seems like it wouldn't hurt anything. I know that the rust is always worse than it looks.

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

First, find and repair the leak.

Rear glass seal, boot/trunk lid seal and antennae mount are the usual suspects. You are wasting your time trying to "band-aid" the problem.

Second, identify the extent of the rust. Flash light, mirror, boroscope, what-have-you.

Third, decide on your plan of remedy.

Don't just willy-nilly pour some in "miracle" sealant expecting it to solve the problem. 90% of the time it aggravates the problem by blocking drainage passages.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

I wouldn't try welding it. Check out the Miracle Rust product from Mercedes Source.
https://**************.com/problems/rust-corrosion/miracle-rust-repair-your-old-mercedes

Don't be heartbroken, it is fixable. As suggested look for leaks and repair them as possible. Drill very small holes inside on any double panels and spray using a straw type nozzle a rust preventive inside such as Fluid Film. Remember anything sprayed inside will need to be cleaned away to be welded to get good welds and prevent fire so take this into consideration on whether to do it now or wait until you can weld it.

Pictures of the car?

Kudos for preserving a 35 year old classic!!!

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

I have gathered the materials to complete such a project in about the same area on my car. The welding on the car's skin is limited to spot welding in one location then welding on another area to allow the previous spot weld to cool before the area next to it is welded.

There is no continuous welding because that would distort the metal and create a lot of body work.

I'm not trying to make it sound easy. I'm trying to point out that the welding done on the metal which will be painted isn't really that difficult. I have a donor car that I will cut and patch until I get the feel of temperature and wire feed.

Anyone who works for a living might be better off paying someone to do this. The problem I have is finding someone I trust. Most body shops spend resources on insurance work and even the best of them refuse to do anything right because of the opportunity cost of throwing something together and selling it to rubes who just want the car to drive and trade in sometime in the near future.

No disrespect to the many fine craftsmen and shops across America. They don't have to advertise, the wait to get them to do anything is long and the price they ask, while being fair, is outrageous, imo.

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