How to weld the front fender on w123

The front fender corners on w123 are usually the first thing to rust on these cars due to the factory seam acts as an rust attractor where water will get in eventually. The easiest way to fix this problem is to simply change out the front fenders. Some years ago they were still cheap, but prices are now rising dramatically in some areas. Also replacing the fenders has some issues as well. #1: They might not fit exactly and you will have to do minor body work anyway. #2: The whole part has to be resprayed in matching colour of the car, this can be quite expensive and the colour might be hard to match exactly.

w123 front fender seam
w123 front fender seam. Common rust attractor.

The second option which is cheaper, but more labour intensive is to fix the old fender yourself by welding. This will keep the originality of the car more if you care about these things. As a bonus you will get rid of that seam permanently too, however this will leave a small sign that it has been repaired. I personally think this seam is an ugly feature of the car and that Mercedes really skipped on production costs when making this panel. I have fixed in total of four front fenders for two different cars with this method and have been successful.


As a condition you need to have a decent skill in welding, this panel is quite difficult to weld for the untrained due to the thin body panel and also the curvature in the corner. It is quite easy to burn through and also forming the metal piece of still can be tedious. However if you are as stubborn like me, then don’t be afraid of this task, use your time and go carefully and you will be successful. (I am no master in welding)

Other stuff you need:

  • Steel 0.8mm
  • Welding tool and accessories (TiG is more suited for this, but a high quality MiG is also fine)
  • Paint of your choosing

It can be done with either the front fender off the car or just leave it on, it all really depends on haw bad the rust is. If you are taking the fender off then you have to be really careful of not scratching the paint which is easier said than done. If you can leave it on the car then it is preferred.

Start by removing the front bumper, it is only secured by two big bolts in the front then two small one at the corners.

Locate the rust, start picking out the flakes and the damaged paint to located the where the good metal is. Then start cutting with the power grinder. Cut at least 1-2cm from the rusted metal to ensure only healthy steel is left.

rust removed from fender
Rust removed from fender

As you can see the hole is much larger than the apparent rust area, everything has to go. Now the hole is defined, then take some sheets of paper, thick paper is better, then use a marker to outline the shape you will need to obtain in order to cut the metal.

Once you have cut out the sheet of metal, the shaping starts, also you will need to fine cut it. so it fits exactly over the whole. This part is where you should use most of your time to get it right, since a right fitting is crucial to get a good weld seam. On this section I used in total 3 separate pieces of steel. One for the big curve, one for the top area under the bumper, then one small one for the bumper bolt hole.

welded front fender
Holes welded

After it is welded you should start polishing the metal to the bone to get id of deposits, then the seam needs to be grind down, use a floppy grinding disk that will follow the contours of the surface more. It is important to not grind too much so there will be holes, remember the metal is quite thin! Any last details have to be smoothed down with body filler. Remember a good weld will minimize the amount of after work with filler, so you should try to make this first step as good as possible.

front fender weld seam grind down
Weld seam grind down

Then the rust protecting base coat have to applied before the body filler. Many use body filler before the base coat, this is not recommended since body filler can attract moisture.

First base coating
First base coating

Start applying body filler to smooth out the last irregularities. When working with car cosmetics the rule is usually greater result with more time spent, but time is precious and only use the time necessary to get the result you want. The area which is going behind the bumper doesn’t have to be perfect since no one sees it. Also remember the back side where all the dirt and stone chips are hitting. This should have a thick layer of paint of any colour and some body protective coating that can withstand stone chips.

body filler
Body filler

Use the sand paper to smooth the filler down and feel with your hand that you get the result you are looking for, you might end up doing this two or three times and a fast drying filler makes things quicker.

Second and 3rd base coat over after filler
Second and 3rd base coat over after filler

Apply the second and third base coat after. Then let it dry completely before you start spraying with the nice top paint. Be sure to use many thin layers instead of one thick one which will start running, then all your work is ruined. The hardest with paining is definitely making an even coating without it starting to run.

Base coat finished
Base coat finished

I couldn’t paint the top coat the same week, so I had to use the car with the paint like this for while. But this is anyway how it looks if you don’t paint your car in the same colour.

So then when possible paint the top colour to make a perfect finish.

front fender painted and finished
Front fender painted and finished

I was quite pleased with the result, no seam and smooth finish, basically no trace of former rust.


Welding the Front Quarter Fenders

Why get new ones when you can use 30hours trying to fix the old ones?!?!

I wanted to weld the front fenders rather than replacing them, even though I have two replacement fenders. Why is that?

  • Welding practice
  • Stubborned idiot like myself
  • Keeping the car more original
  • No need for respraying fenders since the other ones are blue
  • Replacement fenders have hole for antenna, and my antenna is in the trunk

So I got on with that, but I might replace them in the future if I’m not satisfied with the result.

rusted front fender
Front fender rusted in the seam, like on every w123. The other one is the same..

So on to the cutting, and trying to recreate the corner, was quite painfully difficult actually.

comparing the fenders
Comparing a rust free fender with the old for reference

So then I welded and this was the result:

Front fender welded
After the welding…

It looks better than it was, turned out when i fit it on the car I needed to do a massive job with the filler, but after the result was quite nice. Good enough for now, might change the fenders in the future when doing a complete respray though, but then I will get completely new ones without old paint and stupid holes for antennas.

Welding water drains

Water sports in pools within the coupé

I realized that the car have had a small repair on the lower section under the left side rear quarter window due to a parking ding or something in the past. How do i know that? The lower panel had no water drain holes! How is this possible to be so sloppy when fixing a repair? They must have known it would start rusting immediately when the water has nowhere to go every time it rains.

The outer car window seals are never completely rainproof since the windows are made to be lowered. Especially when the windows are lowered when they are wet they drag lots of water into the panels. This is fine since the water can escape through drains through the lower panel. However here there were no such thing like on the right side. Idiots… The result are massive amount of rust in this area, both on the inner canal panel and the outer body panel. Thankfully the welding was quite easy since it involved straight large panels. The big part was removing the panels in the interior and working in tight areas.

Lets get to work, start tearing those inner panels off, hopefully without damaging them and breaking clips.

beyond inner panel at beackseat in w123 coupe
Beyond the inner panel

Do you see that hole behind the seat belt puller? Obviously there was no hole before, no drains remember… But after removing the the rusted metal a huge hole was left, maybe a bit too much? To get access to the inside I had to remove even the window mechanism.

inside panels removed w123 coupe
All panels removed including window mechanism

The hole left was huge, all that lower panel had top be cut off and even the inner panel is rusted out and needed to be removed.

lower panel before welding
Lower panel before welding, huge hole

After cutting the inner panel and welding in a new one it looked like this. I got access through the other hole in channel to paint it, there is a large rubber plug one can open. Needed to recreate the corner there as well, a bit tricky, but very satisfied with the result.

Inside panel welded
Welded the inside panel and canal corner

What remained was to weld the outside panel, paint it, make drain holes and side trim fastener holes. Obviously paint it on the inside as wheel since all the water is going there from the window. Don’t want it to rust again.

lower side panel welded and painted
Outer side panel welded and painted, not that I made the picture before I drilled the drain holes and holes for the side trim.
lower panel finished
Finishing the lower panel, nmow with drain holes!

Now on to the wheel well, It had some rust holes spread out, and the worst part was at the point where the rubber end stop was (if the car bottoms out this will prevent the wheel hitting the upper part). I had to make a new bracket from scratch.

wheel well before welding
Was quite some rust in some areas and needed to remove the old metal and the suspension rubber end bracket

The metal bracket holding the rubber on the left side of the car was so rusted I threw it out. This is a part that is welded to the car and cannot be replaced. So I made a new one, I think my design is even better and more strongly built than the old one. The rubber can be replaced and I changed them too. In the picture under you can see the original bracket for the right side, and creation of a new bracket for the left side. They are mirrored since they are on different sides of the car.

rubber mounts for suspension w123
Making new rubber bracket from scratch.

I also made a new mount on the car out of 2mm steel plate which is super solid. The end result is under

wheel well after welding and painting
Wheel well after welding and painting, notice the rubber bracket is now mounted

Welding the rear quarter wheel arch

Wheel arches mania

Started welding on the rear right wheel arch today, I hope to finish it tomorrow. It is hard to shape the metal to the smooth round shape of the wheel arch. Much easier if I had a replacement part, but these parts are rare and expensive since it’s a coupe and the rear section differs from the station wagon and sedan models. A part from an old wreck are hard to come by and would had the same rust problem in 99% of the cases. So better off doing it the hard way.

rusted right rear wheel arch
Rusted right rear wheel arch

Obviously I had to make a huge hole and remove the old crappy metal.

Welding metal into the wheel arch
Making some wheel arch

Cutting panels and shaping it is an art, and I’m not an artist, just a simple engineer.

Welding wheel arch
Welding first seam

Welding along the seam needs patience, the metal here is very thin.

Welding wheel arch
Welding continues

Also have to make the inner weld which I didn’t take pictures of.

Welding wheel arch
Adding lower part

This last part is a tricky one though. How will I be able to replicate the smooth shape?

Welding inner wheel arch
Welding the inner sides

Making the car water proof once again.

Wheel arch welded tight
Final polish before paint

Welding done! Think I have swung the hammer more times today than any other day in my life! An exact reshape of the arch was not possible from my side. Will need some filler to make up the last touches.

Wheel arch painted
Wheel arch painted but not done

Painting the metal and then applying filler.

Wheel arch finishing touches
Finishing the touches with some body filler to get the smooth shape, harder than I thought. Used a filler that is easy to sand.

I was quite pleased with the result, but is not as perfect as a replacement arch, will look into that in the future…maybe. So fast forward in time, here is the end result. Not bad for an amateur like myself!

Result after welding wheel arch
Wheel arch after painting

You can see the slight difference in color where the old and the new paint meet. I also think the shape of the arch is nice.