85 300D rear wheel well rust repair
 

I was working on some cosmetic paint repair around driver side rear wheel well, when I noticed some suspect spots on undercoating. Started picking away and found two bad spots and a couple of minor ones.

One is on the spring stop bumper (?) - It doesn't look like it ever bottomed out, so structurally I am not too concerned. But it needs fixing.

Other is sheet metal adjacent to where antisway bar bushings are. Metal is solid where bushing housing bolts on, but sheet metal has gone.

Think of several ways of repairing:

1. Take it to shop and have them weld/braze in new metal. Strongest, but then the inner face of the metal repair cannot be painted and some of existing inside coating will be damaged by heat. Outside would be cotaed with POR and spray on undercoating.

2. Coat everything with POR15 and while still wet, rivet or sheetmetal screw on thin sheetmetal patches. then overcoat with POR.

3. Instead of using sheetmetal, grind back the existing coating to bare metal a bit further and lay up a fibreglass patch using either POR as the resin (have done that in past) or perhaps a 2-part epoxy resin.

Well Graham, you have a number if good drivers to choose from, so you are not out of transportation. Since you have found those trouble spots, I would encourage you to really look that underside over really very carefully. I suspect that there is more rust lurking under there then you have discovered. Look at it from the top also. Pull everything out and away from the car floor (carpets and pads) until you have nothing but metal. I wouldn't spend a dime on repairing what you have already found, until you are certain that it is all there is, nothing more. Look at the trailing arms coming back from the sub body mount also. There have been a number of trailing arms giving up lately because of rust. That rust acts so much like cancer. Hard to see at times, and almost impossible to cure once it begins. There is tremendous pressure on that torsion bar especially on high speed curves. That is what gives you control with that independent rear suspension. If it doesn't hold on a highway curve at highway speed, people are going to get killed.

You are right that I don't need to be in a hurry.

One of the problems in doing a proper inspection, is that the entire bottom of the car is coated with oil and grime from past oil/fuel leaks and rustproofing spray we put on our cars up here. In a way it is good because major items don't rust. What does rust, is the inside of cavities that don't get any oil. The car should really be steam cleaned.

I did check the trailing arms a little while ago and they are not rusted. There is a bit of rust around the edge of the sockets that the subframe mounts fit into. But it doesn't seem anything that a little POR won't fix. Having owned this car for 20 years and driving it in all conditions, finding rust is not uncommon. I have tried to keep ahead of it and the car actually looks good.

I am going to check as much as I can on the underbody. Maybe our pressure washer will clean off some of the oil in suspect areas. Any good ideas on cleaning underside of car welcomed!

Never ever ever use a pressure washer on the underside of a car if you are in a damp / wet climate. You will manage to push water into more places where it shouldn't be.

I don't have a quick solution for cleaning the crud off of the underside - I use engine de-greaser and careful scrubbing with a brush. The best way of removing the stupid rubber Mercedes factory fitted undercoating is to use a powerfull hot air gun / blow torch with a rigid paint scraper (the type you'd normally use on window frames).

If the metal is rusted underneath you can easily remove the undercoating with your fingers - if you were to use a pressure washer you'd blast straight through it; possibly past the rust or through it pushing water into places where you can't dry it out. If water gets between the spot welded joints on these monocoque constructions you may as well kiss your car good bye.

Answer
 

That is structural, weld in new metal.

http://www.peachparts.com/shopforum/video-forum/272870-welding-video-threads-auto-body.html

Bodywork - PeachParts Mercedes ShopForum

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Considering that sheet metal that needs adding here would not be accessible from one side after welding, wouldn't the bare metal be likely to rust again from the inside?

The main structural pieces are still good. But the sheetmetal no doubt adds to the structural strength of the heavier pieces. Weld repair would be my first choice, but not if metal can't be protected after welding.

One possibility would be to weld repair and leave some access holes to spray in rust treatment. That is how rest of car is protected once/year but I guess they (Rust-Check or Krown) don't get at everything, especially where MB undercoating has failed.

Answer
 

You access through the bolt holes.

The heavier bits are unibody reinforcement plates.
You need to be aware that the Mercedes unibody is all structural to greater or lesser degree depending upon the location.

http://www.peachparts.com/shopforum/2933567-post8.html

http://www.peachparts.com/shopforum/1074163-post14.html


There are many pictures of structural weld repairs in this thread.
Who has the most rust and still drives
http://www.peachparts.com/shopforum/body-repair-restoration/86054-who-has-most-rust-still-drives.html

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That thread is a horror show :(

Over the years, when I have had body work done by shops that included weld repair, rust has more often than not come back in the area of the welds. Interestingly, for the repair of the spring perches on the W210 (common failure), MB now recommend that the new sheetmetal part be riveted in place rather than welded. Perhaps they have seen the same problem.

I cleaned off the other side rear wheel well. It had been repaired in same place adjacent to bottoming out bumper. ( Probably by me! I have had this car for a long time ;) ). It was done with what looks like glass cloth and epoxy resin, POR and overpainted white. Seems OK, but again, those bumpers have probably never come into use. Added: Checked it more closely, and there is some rust and the rubber bumper had separated, so I took it off. Not so sure I can get that bolt out! Soaking at present!

Going to check other suspect places before deciding on repair method.

Been trying to get the bolt out that holds the rubber buffer in place. Even after spraying rust penetrant in for a couple of days, that bolt won't budge. It is just an allen capscrew. I used a good quality allen key but it turned in the screw's socket, likely ruining it.

Question - Has anyone removed this buffer? The rubber appears to fit over a steel cup washer. Is that washer an integral part of the buffer? This is what it looks like:
http://img.partsbase.net/images/749/749973.jpg

This is other side which should also take off:
http://www.peachparts.com/shopforum/...r-img_1690.jpg

Any suggestions on how to get that bolt out, preferably without ruining thread welcomed!

- There is not much room to drill it out - maybe I need an angle drill?
- Vice grips - Cup washer limits access, but I can flatten it out, But VG will probably chew up the head.
- Heat - may be able to get small torch in there.

Stupid job considering those buffers don't do anything:undecided. Mainly wanting to gain access so I can repair rust.

Answer
 

Cut away the rubber, then use heat, oil, heat, oil, until the bolt comes out.


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Have the rubber cut away on other side, but haven't tried heat yet. Have a feeling the rust will mean that in the end I will have to drill :(

With the holes you have I'm surprised you can't tackle this problem on both sides.

Well, in a way I did ;) I used the holes to spray in the penetrant! Not much else you can do on top side because the "nut" is integral part of the bracket.

I applied some heat on passenger side buffer support bolt, hammered the cup washer out of the way and used vice-grips on the knurled head of the bolt. It actually came out quite easily.

I have a shop that is going to quote me on the weld repair. They haven't looked at it yet, but if the heavier metal is too rusted for a weld repair, then we could do as another owner did, and make a new bracket. Don't know if those brackets are available anywhere (Right one below is the MB one that he cut off other side) Looked it up on epc and the brackets (item 71) are shown. There is another part called reinforcement for rear torsion bar bearing (2 reqd). Would that be the beefed up metal that the torsion bar bushing clamps to? Seems good still on my car.

That's what I meant - penetrating oil to the rescue!

I haven't checked to see if those parts you want are available via Mercedes or not. I'm sorry to report that if they are it would probably be cheaper to get someone to bend some new ones for you. They're not particularly complicated to make... if you want to have a go yourself get hold of some 1mm (or is it 1.5mm there?) thick zinc plated steel. The zinc plating process (the stuff here is called zincor) apparently results in a slightly more maleable steel to work with. The only downside to using it is when you come to weld it - remove the thin thin coating before welding 'cos the fumes (like those on galvanised metal) are dangerous.

I haven't checked with local dealer, but US based on-line dealer wants about $40 per bracket.

I have a shop that will do the welding for $50/hr, so it may be worthwhile buying the brackets if he decides they need replacement rather than repair. I am sure it will take more than an hour to make a bracket!

Anyone hazard a guess as to how many hours for weld repair job assuming same work on both rear wheel wells?