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Discussion Starter · #1 · (Edited)
[This thread was moved from XMAS present from my neighbour - I should never have tried to hi-jack that thread but at the time I wasn't going to really follow up with this post. However, @efair and @elphie did post some interesting comments on it that make it warrant its own thread about how to repair a bumper yourself]
[I've updated this post and reworded it a little to make it more "new"]


My bumper is seriously cracked and scratch and it's quite an eye sore to me. Scratches, at least THESE scratches, I could really live with. They're on the underside of the bumper and the car is so low that you really can't see them.

But the corner is broken off and it makes quite a racket when it flaps around at 200km/h and it's on the side of the car so quite visable! But if I'm going to repair that corner, I can just as well fix those scratches while I'm at it. Not sure how long it'll stay "fixed" (considering all humps etc on the UK roads :) ), but why not.. !?


This one I figure I just put some filler on and then sand down and paint. This is the underside of that spoiler or splitter or whatever it is (goes on top of the actual bumper), so it's not visable unless one shove ones head UNDER the car :). It'll be a quick-and-dirty job, that I can do myself.
Hood Automotive tire Wood Bumper Automotive exterior


However, this is the sections that could need some plastic welding I guess.. ?
Automotive tire Automotive lighting Hood Motor vehicle Automotive design
Automotive tire Road surface Asphalt Grey Floor


Ok, so this one is underneath, but it IS quite a big crack, so I'll need to fix that one as well.
Wood Gas Font Engineering Automotive design


This is the really bad corner! It HAVE to be fixed!
Automotive tire Wood Bumper Hood Gas


Other side isn't as bad, but still need some welding. The scratches I can give the same treatment as that black whatever-it-is, splitter? But that little corner there need some welding..
Automotive tire Wood Trunk Fender Automotive wheel system
Rectangle Wood Tints and shades Art Composite material
 

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I would definitely try to repair that... commence thread hijacking in 3, 2, 1 (if caught by the moderators, start a new thread)

When repairing cracks in your bumper cover, it's assumed you will be repainting - at least in the area of the crack. In your case, you'll probably want to repaint the entire thing.

First dumb question: is that the factory AMG bumper cover, or an aftermarket product? I've only worked on the pre-facelifted R171 factory covers, not the facelifted, and mine didn't look much like yours.

Second dumb question: Somewhere inside the bumper it should be stamped with the material it's made of - in my case it was stamped PUR which stands for polyurethane. Is yours also stamped PUR somewhere? This is so fundamental that you should probably not read any further until you've answered this question!

PUR is an older material and cannot be "plastic welded" or "heat stapled" like many newer covers. Many body shops won't work on PUR because it requires a little more labor to repair. I found it to be easy once you understood a few basics.

I use Black JB Weld Plastic Bonder (a two-part epoxy), a roll of aluminum duct tape (super sticky and just the right amount of flex), a small wire brush, and a dremel tool with a 953 stone. I tried a few other epoxies and this was the only one that stayed flexible after it set (which is important given the environment). And the color is important - you will see why below. See Plastic Bonder Syringe - Black and Dremel 953 Aluminum Oxide Grinding Stones - dremel.com . You will probably need at least three or four syringes of epoxy. The aluminum tape is used to hold the things together while preparing and epoxying.

I suggest starting and practicing with a smaller crack that is out of sight - so you can get the hang of the process before you start working on really large cracks in really obvious places.

First pass: Working crack by crack, position your bumper cover so that the crack fits back together "naturally" - as close as humanly possible. If you have to, use pillows or pads strategically to ensure this fit. Apply aluminum duct tape to the >>>outside<<< of each crack to hold things in this natural position temporarily. Then on the >>>inside<<< surface, clean the area of the crack with a small wire brush and then grind along the entire length of the crack with the tip of the stone, leaving a v-shaped groove with a target width and depth of 1/2 the thickness of the material. Don't worry if you go 2/3 through, or even penetrate fully (but try to avoid that - you'll get the hang of it quickly). Wipe away all the grinding dust, mix some epoxy and fill this inside v-shaped groove with a good bead of epoxy. Don't be stingy here - allow the bead to "overflow" the v-shaped groove and bond to the surrounding material - this is what gives the repair the strength to survive. Let that epoxy set thoroughly overnight and then repeat the above with the next crack(s). Pro tip: If you have several cracks in the same area, prepare all of them first and then apply epoxy to all of them - you'll waste less epoxy this way, and the repair will go quicker.

Once you've laid down an epoxy bead to the >>>inside<<< of all cracks, you can remove all the tape and turn your attention to the >>>outside<<< surface of each crack. Because this is the visible surface, you'll want to be a little more careful here. Again working crack by crack, grind a similar v-shaped groove on the >>>outside<<< of each crack - along the full length of the crack again - but this time try to grind just deep enough to expose that first bead of epoxy - it will appear as a thin black line as you grind from the outside. Once you've ground all cracks, apply a much smaller bead of epoxy to each crack - just enough that the epoxy is slightly proud of the outside surface. Allow this to set thoroughly. Once set, you can sand the outside surface flat in preparation for painting.

If you're repairing cracks in a >>>black<<< bumper cover, I would recommend you use the tan epoxy, not the black.
 

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Discussion Starter · #3 ·
When repairing cracks in your bumper cover, it's assumed you will be repainting - at least in the area of the crack. In your case, you'll probably want to repaint the entire thing.
Since it's underneath, I was going to try to only spray that part. I got a (small!) spray can from MB in the right colour and I figured it wouldn't have discoloured that much in eleven years. If that doesn't turn out good, then I guess I HAVE to respray the whole thing..

First dumb question: is that the factory AMG bumper cover, or an aftermarket product? I've only worked on the pre-facelifted R171 factory covers, not the facelifted, and mine didn't look much like yours.
As far as I know, it's factory. And considering all the dirth and such in "hidden" places, It's been there a VERY long time :).

Second dumb question: Somewhere inside the bumper it should be stamped with the material it's made of - in my case it was stamped PUR which stands for polyurethane. Is yours also stamped PUR somewhere? This is so fundamental that you should probably not read any further until you've answered this question!
It's stamped ">PUR GF 17<" (might be a CF, hard to tell).

PUR is an older material and cannot be "plastic welded" or "heat stapled" like many newer covers. Many body shops won't work on PUR because it requires a little more labor to repair. I found it to be easy once you understood a few basics.
Thanx, that's somewhat bad news, but I think I understand exactly what you're talking about regarding doing it myself. Thanx!!
 

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Discussion Starter · #6 ·
So I've currently ordered a bunch of stuff from eBay (don't shop Amazon any more for various political and personal reasons) and they should hopefully arrive in the next few days or so:
All-in-all £98 including S&H.

I'll update the post with pictures once I got everything and can start repairing the big ones. I'll start with that long crack underneath the corner, just to .. "warm up" to it. If I mess that one up, it's not the end of the world. But if I mess up that corner where it's sheared off, then things will be much, much worse!
 

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Discussion Starter · #7 ·
Just as a picture of what it looked like straight of the car. That long, black one is the whatever-its-called, splitter or whatever. It goes on top of the bumper.
Grass Automotive exterior Font Auto part Machine


Gadget Wood Office equipment Material property Audio equipment


I didn't want to take apart anything else, so the rest will have to stay. UNLESS (!) I'm forced to repaint the whole thing. I'm hoping it won't come to that. It's a bit pitted (not to bad though) and there's some asphalt residue here and there, but I'm going to live with that.

I don't want to scrubb with a Scotch-Brite to much, because the material is quite soft and I'll just end up scratching it more than it already is :).

Once I'm done with the repairs and covered it up with the spray paint I got from MB, we'll see how it looks in sunlight and I'll make the final decision then.
 

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I agree with avoiding scotch-brite. You should use a good degreaser though, it wouldn't hurt to do the entire thing, outside and inside.

Take your time and you'll have good results.

Post some close ups of the inside of the worst damaged area.
 

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Discussion Starter · #9 ·
The back/inside of that long crack underneath. I'll call that "The Somewhat Good Crack" :).
Hood Wood Automotive exterior Twig Tints and shades


This is "The Bad Corner". Might be a bit tricky, but shouldn't be TOO hard to do.
Wood Automotive exterior Gas Tints and shades Wood stain


That corner edge is also cracked off but that'll be a bit tricker to do, it's in a very ugly place..
Wood Beige Flooring Tints and shades Composite material


This is "The Good Corner". That little piece actually came of by itself from all the handling :).
Wood Flooring Automotive exterior Wood stain Hardwood


I've used some heavy duty degreaser, power washer and finally some "Daemon Clean" to get most of it off and it DID cleanup quite a bit.
Motor vehicle Wood Automotive design Engineering Automotive exterior
Automotive tire Motor vehicle Bumper Automotive exterior Gas
 

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BTW have you used these kind of two-part epoxies before?

Having a good work surface, where you can re-orient the work as needed, will make this easier. The bumper cover is not heavy but it is very bulky and flexible, which for some cracks will work against you. JBWeld being liquid(ish), you'll have to orient the work so it stays put at least until it sets (turns solid), and in many cases until it cures (fully "hardens"). I had two sets of sawhorses and a half sheet of plywood to work on, along with a good collection of old pillows.

If a crack is in a "high flex area", and you pick the work up before the JBWeld >>>cures<<<, the repair will fail. So be patient, if you're not sure, let it sit.
 

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Discussion Starter · #11 ·
BTW have you used these kind of two-part epoxies before?
I have. Thirty or so years ago :).

JBWeld being liquid(ish), you'll have to orient the work so it stays put at least until it sets (turns solid), and in many cases until it cures (fully "hardens").
Oh.. No, that I never worked with. Only the more common whatever-its-called, the one that have always prided themselves that it holds lifting a car..

I'll see what I can do with a proper workbench or holder or something. Thanx for the heads-up!

If a crack is in a "high flex area", and you pick the work up before the JBWeld >>>cures<<<, the repair will fail. So be patient, if you're not sure, let it sit.
Understood and duly noted! Thanx!
 

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Maybe I've had too much coffee and I'm micromanaging your repair too far in advance... but a few things: 1) it's good to have a stack of index cards and a box of toothpicks when using epoxy like this. 2) the syringe includes a cap - open it, dispense a puddle of sufficient size onto a card, then put the cap back on. 3) Stir the puddle with a toothpick to mix the two components thoroughly - the result will be about the consistency of ketchup. Then use the toothpick to dab the epoxy into a bead shape in the V-grooves you ground. For the first beads, on the inside, it's generally best to not move the bumper until the epoxy cures. Throw the card and the toothpick away and start on the next batch.
 

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For your bad corner:
Food Ingredient Recipe Wood Cuisine


You will need to support the bumper so that this crack is "closed" before applying aluminum tape to the outside of the crack, and continue to support it while grinding and epoxying so that the two sides stay together.

If this comes apart after setting, I would try again - grind away the epoxy but this time put the mesh in place over the bead, and then apply a copious amount of epoxy over and around the mesh.
 

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Discussion Starter · #14 ·
That corner will be the last one I do. I should have had plenty of practice by then :). I'll see how good that aluminium tape I got is, but I'll see what I can do to support it properly. I have a few wood clamps, they might be usefull somehow.

Thanx for the heads up.
 

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Discussion Starter · #15 ·
So that splitter-thingie is getting ready.


Sanding down any high points. It's not as horrible as it first looked.
Automotive tire Wood Hood Road surface Flooring


Filler and sanding.. Smooth as a babys bottom! :)
Tire Automotive tire Tread Wood Wheel
Hood Wood Bumper Automotive tire Automotive exterior


Primer..
Automotive tire Wood Synthetic rubber Rectangle Flooring


First coat black..
Automotive parking light Hood Bumper Automotive exterior Auto part


Second coat..
Hood Bumper Automotive tire Automotive exterior Grille


I'm going to take a good look at it tomorrow in daylight, MAYBE need a third layer. But then the clear coat on that and it will look better than new :).


That U-POL paint is absolutely amazing!! I used their Etch Primer for my brake shields and calipers and it sticks to absolutely anything it seems! This one isn't Etch, but it was awesome none the less. AND it dries in 20-30m. I gave it about an hour, just to be sure, but it was more than enough! I'm never using anything else.
 

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Discussion Starter · #16 ·
Some light sanding with a 800 grit wet sandpaper and then an additional three layers of paint and probably six or seven layers of clear coat and I'm quite happy!

Ok, so I'm not a painter! :D :D. But considering that I don't have access to a proper paint booth (i.e. no lint or dust in the air!), I think I did quite alright. I'm more than happy with the result!

Before (refresher):
Tire Automotive tire Tread Wood Bicycle tire


After:
Automotive tire Wood Automotive exterior Asphalt Road surface
Automotive tire Automotive lighting Rectangle Bumper Wood


Looking VEEEERRRRYYY closely, and it's not perfect. Had I payed top dollar for a professional to do it, I'd send it back. But... This is going to be next to invisable once its on the car and I've driven it on dusty and wet roads :).
Hood Netbook Automotive lighting Automotive exterior Communication Device

Material property Gadget Tints and shades Composite material Electric blue



I'm supposed to get the rest of the stuff I need this week and then the REAL fun begins :).
 

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Pics are hard to "review" on a monitor as the setting is never the same but did you sand between each and every coat? I ask cuz it looks "thick"??
 

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Discussion Starter · #18 ·
Pics are hard to "review" on a monitor as the setting is never the same but did you sand between each and every coat? I ask cuz it looks "thick"??
No, I didn't. I was considering it but I kind'a wanted the tick look.. Gives it a bit more .. depth and shine.

Although, in retrospect, I probably should have done that. Lessons learned for next time.
 

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Discussion Starter · #20 ·
I've gone over it with some white spirits, trying to remove all stains, tar/asphalt pits etc and it is "somewhat bad". It's not horrible, but I'm now exactly ON the fence regarding a complete respray.

Before, I was quite a bit from that, planning on only doing those two "wings" underneath in the corner. But now...

Unfortunately, it's not very obvious on the photo. But in good light and looking at it very very closely, it IS "somewhat bad". Quite a lot of stone chips (fairly small ones luckily, but there's quite a few of them). So I'm going to get some quotes and see what they say.. I'm not going to spend MUCH more on this..


I can do a lot of the base work myself (most of it is already done - remove from car and dissasemble :) ), but the sanding is about as much more I can do. Not sure if they want/need it primed or so, so I'll leave the rest to them. And the sanding of each laquer coat..
Automotive tire Bumper Automotive exterior Automotive wheel system Rectangle



But does anyone know how to remove those black "cover-wings"?
Hood Automotive exterior Wood Automotive tire Bumper


There's no clip or anything and if I had to guess, they're glued. But how hard? I don't want to start prying and risk cracking something. The bumper is VERY soft! Those black wings are made of the same material as that other big "splitter-thingie" that I just repainted, just much smaller.

The "splitter-thingie" is stuck in place with a bunch of clips (the three top one is part of the grill fastener clips), but these two smaller ones are not..
 
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