Restoring an M3's damaged carbon roof: My experience

[GiorgioE82]

Just like many of you have experienced, my 2012 M3’s carbon roof was in pretty bad shape. I purchased my car back in 2020 and one could tell the roof was on its way out. Other maintenance items had higher priority so this kept getting pushed back. However, after many delays I finally decided last week to just go in and do it no matter what the outcome. TL; DR…it came out rather great! World’s better than before, even if not perfect. I had a rough time just searching for DIY articles, videos, forum posts that could show enough detail of the actual process; hence there was a bit of a learning curve which led to some minor mistakes. I was able to fix most of them even though the end result was not perfect. If I were to do another one, I am absolutely sure it would come out great after knowing what I know now.
Since some of you may be also looking for some more real-world experience on the subject, I thought it would be good to do this thread and explain is as much detail as possible: What I did? How did I do it? What would I change?
Disclaimer: This is for recreational purposes only and you are responsible for doing your due diligence in researching the topic. Anything I mention here, if you follow, do so at your own risk. I am NOT responsible for any damage, issues or anything like that. If you mess up, you’ll just have to figure out how to undo the damage or start over.

With those formalities out of the way, I’d like to present what I feel is the most logical way of breaking down the process:
I. Problem Summary
II. Materials used
III. Timeframe
IV. Old Resin removal
V. Resin test
VI. Steps for resin application (including wet sanding)
VII. Steps for clearcoat application (including wet sanding)
VIII. Lessons learned summary


Bear with me if it takes me a bit to write the whole thing as I’m doing this in whatever bit of free time, I have available.
Also, I'm not sure if one can lock threads here, but if you see it locked its because I want to add as much content to it as I can before publishing each post. Hopefully it's not much of a mess!

Having said that, let’s get to it!!

[GiorgioE82]

I. Problem Summary

Let’s face it…the carbon roofs look cool AF. But if you’re like me and live in a hotter climate, plus your car spends a lot of time outdoors, you have run into this problem. Mainly, the carbon roof starts to look yellow in areas, then looks flat, then you can see the carbon weave through the top coat, and then inevitably blisters and bubbles begin to appear. These “bubbles”, when torn, reveal the underlying carbon fiber. Also, there are instances where rock chips accelerate the bursting of these bubbles or they outright tear through the top coat.
So, what is happening? In essence, the resin in the carbon fiber is breaking down and degrading due to UV exposure (I’ve also heard some people have had issues with harsh chemicals). I’m not completely sure, but BMW either uses a very poor and thin clearcoat on top of the carbon or does not use anything at all. Again, I do not know for certain which of those is true. But what I do know is what it looks like: flaky, powdery old resin messing up the look of the roof.
In some bad cases, like mine, the issue is so bad that you can actually start to peel off the top layer of resin/clear with a razor blade and the rest will peel by hand. In other cases, if you start when there is enough resin, you may need to sand off most of it for prep.

II. Materials used
a. Two-part epoxy resin. This can be found online or in most automotive paint shops. My wife does hand crafting with resin and she buys hers from an arts and crafts shop. I decided against using this type as I wanted something more “industrial”, for lack of a better term. There are many brands out there and each comes with two liquids: the resin and a hardener. Each manufacturer will specify their mix ratio (either 1:1, 2:1, even 4:1). ONLY follow the instructions specific for your product!!! Kits come in multiple sizes; I’d recommend getting the one that comes in quarts. I had to do with the gallon kit since that is what was available for my specific brand here. For the record, I used the ULV300 epoxy resin kit from Fiberglass Coatings Inc. It comes as a gallon of resin and half gallon of hardener. (see pic).
b. Clear Coat. I went for a 2K Clear version mixed in the can by one of our local auto body stores. It is packed in a can similar to Sp*****x but filled with their product (a clear better suited for our climate here). If you have access to an HVLP gun or paint booth, even better. I used 3 cans of clear.
c. Miscellaneous clear plastic mixing cups. These are to mix the epoxy and resin. They’re cheap and can be found at most hardware, paint stores.
d. Silicone mixing paddles. For mixing the epoxy. Buy a multi pack. You’ll need it. They’re cheap.
e. Sandpaper, or sanding discs. In the following grits (120,220, 400, 600, 800, 1000, 2000, 3000).
f. Nitrile gloves. Things get messy, don’t skip these.
g. Respirator. For use when mixing and putting down epoxy, sanding and when applying the clear. I bought a 3M one for about 30 bucks off of Home Depot and it worked amazingly well.(see pic)
h. Safety glasses. Cheap insurance for your eyes against errant sanding dust, epoxy, old resin shavings and clearcoat fumes.
i. Single edge Razor blades. For peeling off the old resin.
j. Auto trim removal tool. For removing the antenna, and the roof side rails.
k. Acetone. You’ll need to wipe off residue with it.
l. Microfiber towels. Go to Costco and get their 36 pack.
m. Paint brushes. For applying the resin.
n. Masking tape and plastic sheeting. For masking off the area and protect the car from overspray.
o. Machine polisher and accessories. Yes, hand polishing may also work but having a polisher (in my case a DA polisher) helped to speed up the process quite a bit. Don’t forget cutting compound and polish (with their respective pads).
p. Pump Sprayer or Spray bottle. You’ll be constantly spraying water during the sanding stages.
q. Roof rail replacement clips. You know BMW plastics are brittle and (just like I did), you may end up breaking a few of them. Note: BMW has a ton of different plastic clips. The correct part number is:51138045321. They come in a strip of seven pieces and have adhesive backing. About $25 for the strip of 7.
r. Double sided 3M red tape. The same stuff used for spoilers and such. To reattach the antenna to the roof after all is said and done.
s. 1:1 isopropyl alcohol/water mix or automotive paint prep. To clean up between stages in the final wet sanding, compounding and polishing phase. Fireball Reborn, Gyeon Prep, CarPro Eraser will all work.
t. Resintint (optional). To add tint to the epoxy. I’ll explain why in the description of the steps.
u. Air compressor. To help in blowing stuff out of the way, and dry out the roof between sanding stages.


III. Timeframe

The whole thing took me about 4-5 days. BUT bear in mind, that a lot of that time was waiting between curing stages of either the resin or the clearcoat. ACTUAL work time was along these lines:
- Peeling old resin, removing roof side rails , antenna: 3 hours
-Wipe down and masking:1 hour
-Initial sanding: 1-2 hours.
-Actual epoxy application 1-2 hours (with 4-5 hours in between for cure between layers)
- Epoxy Sanding: 2 hours
-Clear coat application: 2.5- 3 hrs ( 4-5 layers of clear with 20-30 mins in between each layer)
-Final sanding and refining, unmasking, antenna and roof rail install: 4 hours.

[GiorgioE82]

IV. Old Resin removal

Like I said, my roof was pretty beat up. And I had purposedly let it become worse as I knew removing the old resin topcoat would be easier because of this.
Before starting the process, remove the roof side rails using the plastic trim tool. Be careful. You will more than likely break a few clips. Start at the rear end of the rails and work your way towards the front with the tool. Make note of missing or broken clips to be replaced later.
Remove the antenna. It is held down by double sided like adhesive tape. Just carefully pry around the bottom of the antenna and start lifting it, carefully working your way around it. The antenna has three locating plastic studs so be sure NOT to force anything or you’ll mess them up. Put the antenna to the side. Now, I’ve seen tutorials where people actually disconnect the antenna from the cables in the roof that go to the inside of the car. I did NOT feel the need for this as the area immediately surrounding the antenna body will be covered anyway, and you can just mask the crap out of the antenna with masking tape (which is what I did!).

Pro tip: since your roof rails and antenna are off now, it is a good time to give them a good wash and take care of any water spots or similar. I polished mine with my FLEX PXE80 3 inch polisher and ceramic coated them afterwards, since they were easy to do.

With a razor blade tip, or a pick tool, CAREFULLY poke a small hole in the resin. If you have spots where it has already exposed the weave even better. Just angle the blade so it presses the carbon very lightly and carefully slide it underneath the old carbon weave (see pic for an example... Yeah that's my kid helping me ��).

If you’re lucky, you’ll be able to peel large sections at a time. (again, see attached pic) Otherwise it is a slower process but it really isn’t that bad. Continue until you have done the whole roof and don’t forget the edges. Make sure you reach at least a few millimeters past the roof edge and towards the roof rail channel to ensure that everything will look good once the rails are reattached.
*Note: wear long sleeves if at all possible during this stage. You’ll find yourself leaning into the roof especially In sections that you have already done. Your arms will contact the old resin and your skin may not like it. I know I had a light rash and some itching later on both arms due to this. So beware!
*Note: Both my windshield and rear glass seals were in pretty bad shape so I removed them completely as I had purchased replacements. It’s up to you how you want to deal with yours but my suggestion would be to buy new glass seals as the ones in these cars go bad anyway.

[GiorgioE82]

IV. Old Resin removal (continued)
Your roof may look something like mine in the 1st pic below after all the scraping.


It is now time for sanding and cleaning off the old resin residue. In my case, I did a thorough wipedown of the surface with acetone on a microfiber towel. This got rid of some of the stained spots and made it look pretty uniform. After this I took 220 grit sandpaper to the roof and started scuffing (without too much pressure that it would damage the fibers). I also followed the advice of another thread and after dry sanding a bit did wetsanding . This allowed me to see the color of the residue. If your residue is black you’re scraping away carbon so be aware of that. Try to level it as best as possible without going too deep in the fibers -you do NOT want that. But rather so that the surface appears mostly level. Once done, repeat the wipedown with acetone. You can also use water with just a drop of dish soap for added lubrication during wetsanding.After cleanup the roof looks like the 2nd pic.

Note: At one point during this step, I had several “runs” of milky liquid (which I’m assuming is old clear mixed with resin) that dripped across the roof and made it look stained. This was easily removed with the later acetone wipe. It concerned me a bit but yeah it was gone after the wipe.

[GiorgioE82]

V. Resin testing
Depending on the type of resin that you have, once it is dry it may be clear or it may be other color (more than likely amber). Since I didn’t know for sure how it would dry, I set up a test cup of resin mix. I let it cure in the cup and examined it. As you can see, mine dried out quite amber/yellow. Since I wanted to be sure that this was not reflected in the final layup, I decided to tint the epoxy. There are several ways to tint epoxy (mica powder, alcohol tints, etc.). I chose to user a product called Resintint (again, since my wife had it in her arts and crafts chest). I’ve added a pic below of what it looks like. I chose the black tint for the project. My estimation was that I wanted enough tint for it to start looking somewhat light gray. For my test mix (at 2:1 ratio of 2/3 cup epoxy, 1/3 cup resin) I added about 6-7 drops to the epoxy mix. It is important that you first mix the epoxy and hardener and after that is done THEN you can introduce the tint. Otherwise, my understanding is that the tint will interfere with the mixing process. Again, I let this dry out and you can see what the color looks like in the end. Much different than amber.

Note: During your testing , keep an eye out for the time between finishing mixing and the time when the mix begins to "gel up". Make a note of it as this will serve as a guide for your actual available working time when doing the application.

[GiorgioE82]

VI. Resin application
Ok, on to the main point of this. Full disclosure: Initially my plan was to just slather top with clear coat and layer it up. But after reading up some more and seeing the condition of the CF, I decided against doing just clearcoat, but rather do resin and clearcoat. For lack of a better term, the CF was “dry” and spraying it with clear over clear over clear would just allow the CF to suck up the clearcoat and never really settle nicely. I had read other posts where people had indicated that a couple of months after blasting it with clear they had to redo it. So instead, on with the resin to build up that top layer and then later we can clear it.

Use 1.5 inch masking tape to mask around the roof, making sure you allow yourself room to coat the side of the roof as they go into the roof rail channel. Try your best to cover the channels on top of the windshield and back glass. You may even choose to completely mask the rest of the car using plastic sheeting. It is up to you. However, in my case, I did the full car masking just before clearcoat. Also make sure you mask off the antenna piece as well.

As mentioned before, FOLLOW THE INSTRUCTIONS for mixing your particular brand of resin with the appropriate ratios. Make sure you have your gloves, glasses and all other PPE on for safety. Based on the resin testing done before, mix it up thoroughly for a good 2 or 3 minutes. Add your tint and mix some more until the color becomes uniform. Now, you’ll see bubbles in the mix. You do not want those bubbles in there when applying. There are several methods to get rid of the bubbles and most of them involve heat. The method I went with was having a bowl full of hot (but not boiling!) water larger than my mixing vessel. After mixing, place the mixing up inside the larger bowl (again, carefully due to the hot water) and wait a minute or two. You will see all the bubbles from the different parts of the cup move towards the center of the mixing cup and just congregate there, leaving the rest of the cup with no bubbles. You’re good now!
Start a timer so that you are aware how much working time you have available before the resin starts to gel up. Keeping in mind the working time you have for your resin, grab a brush and your mixing cup and pour a bit of the mix on top of the center of the roof. Do not pour a lot because it will run all over the roof and make it more difficult to manage. Just work in small sections (maybe 1/8 of the roof at a time). Once poured, start brushing a nice thin layer of resin. Continue doing this throughout the whole roof until done. Be as smooth as possible with the brush, as this will minimize your sanding efforts later on.

That’s it. Your first resin layer is done. You should have something that looks like the pic below. Now go on and do something else for the next 4 hours or so. Once the four hours are done, come back and do a second layer on top following the exact same process.

Note: I could tell the resin was reaching the end of its working time as the mixing cup started getting increasingly hotter up to the point where it started becoming a gel (where even slight vapor could be seen). Be aware of this as you don’t want to apply it just as it is becoming a gel, or even worse, after it has become a gel. Believe me, I made this mistake and paid the price later on.

Note: I used 4 hours as my time between layers due to my schedule. I’ve seen posts and videos where the waiting time between layers can end once the first layer is “as tacky as masking tape”. But for me, I waited 4 hours. You do you.

Note: I've read that you can use a heat gun SPARINGLY and CAREFULLY to warm up the resin application just after you're done with each coat , to minimize any additional air bubbles and the like. I did not do that personally, but in hindsight it seems like a good idea for future work.

[GiorgioE82]

VI. Resin application (continued)
After my second layer, I let the car sit overnight before beginning with sanding. This was due to other commitments taking up my time.
This is where you will sand off the excess from your resin layers in an attempt to make it as smooth and flat as possible. In my case, I made a mistake where I let a bit of epoxy that had just started to gel get on the roof. And this made a bit of a gooey mess. I was able to somewhat level that section (about 2 inches by 8 inches right in the middle of the roof). Once the resin dried, though, you could see the wavy lump of excess resin in the middle. Hopefully this doesn’t happen to you, but if it does, you just have to take your time, sand it and level it out.

For this stage I started dry sanding with 220 grits with light pressure to take off the easy stuff. After that I went progressively lighter in grit (320, 400, 600, 800) using tons of water for wet sanding. The importance of doing mostly wet sanding here, is so that you can tell by the color of the liquid being sanded off that you are not back to the level of the CF itself. The resin while being removed should look grayish (because of the tint) or milky. Easy and steady does it.

After each grit, use a damp microfiber towel to wipe off the residue and the compressor to dry out the area and check your work. You should be seeing the CF pattern with no obvious high spots (too much resin and it will look like a lump) or low spots (not enough resin and it will look like exposed CF weave). In my case I had to do a ton of sanding on that center section to get rid of the excess but I was hesitant towards the end and didn’t flatten it 100%. Once you’re up to 800, clean everything, dry everything up.
At this point you want to use your panel prep or alcohol mix and give the roof a good cleaning, ensuring there are no major marks, streaks and such. This is to get ready to apply clearcoat. In the end, your roof should look as in the pic below.

If you haven’t done so already, go ahead and fully mask the rest of the car using plastic sheeting. If you’re doing this in your garage, make sure you park your other cars outside and don’t leave anything around which can get overspray on it.

[GiorgioE82]

VII. Clearcoat application.
Before doing anything else, make sure you have gloves, safety glasses and your respirator (and other needed PPE) on. You don’t want to ingest the clearcoat aerosol!!
In my case, like I mentioned, I used a 2k like clear coat provided by my local auto paint supply store. The process was the same as the Sp*****x2k clear mixing process. Remove the red plunger tip from the can’s lid, and use it to push the pin at the bottom of the can. There are plenty of videos on YouTube about this but my preferred way was to put the red piece on a level, sturdy surface, and then push down the can hard. Once the hardener is released into the clear inside the can, spend a good minute or two shaking the can. Find a piece of cardboard and spray a test pattern on the cardboard so you know what to expect.
Now, in fluid motions and overlapping each section by at least 50 percent, start applying a first coat of clear. It doesn’t have to be heavy. You just want to make sure you get decent initial coverage. Keep the can about 4-6 inches from the roof and work in side-to-side motions. Once you are done, set a timer for 15 minutes and go do something else.
After 15 minutes come back and do a second, heavier coat (check first that the initial layer is mostly dry to the touch). Remember to shake that can!!! Try to keep the application wet without letting it become so heavy that it runs. It’s tricky with the spray can but you’ll get the hang of it. Set a timer for 25-30 minutes.
Come back and inspect the 2nd coat to verify it has dried for the most part. I did 5 or 6 coats (can’t remember) to allow myself some wiggle room when doing subsequent wet sanding. After 5 or 6 coats, leave overnight to dry.
24 hours later
The clear should be super dry to the touch. Inspect the finish. If you are lucky and your conditions and technique allowed it, you should have a pretty decent finish without too many defects. More likely though, you’ll see several places where the clear has texture /roughness/orange peel and even some nibs or fisheyes. Not to worry, that’s where wet sanding comes in!
In my case, I had quite a bit of texture so I had to start with 800 grit as my first sanding step. Hopefully, yours won’t be that bad and you can get away with using 1000. Regardless, use your pump sprayer or spray bottle and spray a good amount of water on a section of the roof (I recommend one of the corners…maybe a 2 ft x 2 ft spot) which you will use as your test spot for your process. Go ahead and wetland with 1000 grit (DA polisher works great here; this is what I used) doing even passes. You should be able to see the light white milky froth of clearcoat being sanded away. Stop periodically, wipe the area with a microfiber towel and dry with the compressor to gauge your progress. You should start seeing a smooth, although dull and not shiny surface. If you see some fisheyes/pockets in the finish, my recommendation is to sand that spot carefully and specifically with a small piece of sandpaper and not the whole block or disc. At least to minimize its appearance.
Continue the process, this time with 2000 grit. Take the same considerations as before regarding inspecting often. Then continue the process with 3000 grit.
Compounding
Since besides being an electrical engineer, I also am a detailer, I own a ton of polishers. Hence, I pulled out one of my trusty DA polishers and a Rupes Yellow Wool pad to use in compounding. The good thing with the Rupes wool pads is that they run cooler than say microfiber or foam for a given effort. I used Menzerna 400 cutting compound for this stage.
Before compounding, go over your section with your alcohol or panel prep mix and a microfiber towel. Set your machine for a speed in its middle range (In my DA, I used midway between 3 and 4 setting) and start compounding. Go over your test section with overlapping passes, no more than a minute or a minute and a half depending on your compound and how fast it breaks down. Wipe off the compound residue and inspect the finish. You should see a marked improvement in shine from the 3000-grit step, even though the finish may be hazy.
For Polishing I used a red Oberk polishing pad with Koch Chemie Fine Polish. Follow the same steps as for the compounding in your test section. After wiping off the polish residue your finish should be quite glossy! Finish off by wiping the section with your alcohol mix/panel prep. Then move on to completing the rest of the roof.
Note: Once I moved to a larger section of roof after my test spot, I noticed that after the compounding stage the section was still quite dull. I took some time to rethink this and realized I had been too conservative in my 1k and 2k sanding and hadn’t removed enough clear. So, I made sure to redo that section and watch for the characteristic milky residue to show up before moving on to the next step. Once I did this, I was able to compound and polish and get a good finish.

See the pics below for the progression of wetsanding->compound->polish. I got asked by some friends what was the "stain" or defect in the pic after the polish stage;that's actually a reflection of a stain in my garage's ceiling!

[GiorgioE82]

Final results

These shots show the quality of the finish at the end. I'm quite happy about the results as it was a dramatic improvement from the mess I had! I'll see if I can take a picture that shows the one defect I couldn't fully remove (the "waviness" in the middle of the roof due to excess epoxy) and upload later this week.

I'll also put up a final post with my "lessons learned".

[GiorgioE82]

VIII. Lessons learned

There are MANY lessons that I learned throughout the process; if I were to tackle a similar project in the future I’m positive I’d see tremendous improvement over the end result of this one just based on these. I’m listing them below in no particular order. Also, I’m sure I’ll think of other things as days go by, so I’ll probably edit this one post to reflect that.
• Be obsessive about masking properly. Initially I masked around the roof rails and front and back glass while doing the resin and then I masked the whole car only prior to clear coat. If I were to do it again, I’d do a really good overall masking job for the whole car. The issue is that I ended up having some drips of resin on the paint (which were easy to take out with a razor blade once it dried) and what was worse, resin inside the glass surround. Most of it I was able to take out quickly but some of it became problematic as it dried WITH the rubber surround. So yeah, mask the crap out of it from the get go.
• The resin will (mostly) self-level. I used a plastic scraped to spread the resin thinly on the roof. This led me to lacking some control in the actual application, which then led to some of the resin spilling over the windshield trim and other places. Rather, pour some resin on the roof and use a wide brush to apply it to the roof. Any brush marks can be taken out during sanding.
• Mind the time. Pay attention to when the resin starts to gel. You do not want to apply past that point. One thing that I noticed was that, as you got closer to the moment it started to gel, the plastic cup holding the resin mix became increasingly hotter (as the reaction was taking place). Pay attention to that as an extra guide.
• Mind your PPE. This should go without saying. I used safety glasses, respirator and gloves for most of the process. However, during the initial scraping and removal of resin, I positioned my bare arms a lot on the roof. Big mistake. I was itching like crazy for the next two days. So, use long sleeves!
• Buy the clips. It’s not a matter of “IF” you’ll break some roof clips but rather “how many” will you break. As with many other BMW plastics, these things become brittle and almost impossible to remove without breaking.
• Use a machine polisher for sanding. I can’t overstate how much simpler the job was when sanding with the machine versus by hand. Make sure you buy enough of the appropriate grit sanding discs. I switched between a 3” and a 6” DA polisher to do most of the job.
• Heat your resin. After applying each resin layer, use a heat gun in low setting to carefully heat the roof surface. This will aid in removing any air bubbles in the resin application. Again, be careful and do not apply heat directly to any one spot. Keep the heat gun moving.
• Proper finishing. Although stated earlier, make sure you do a proper small test spot when starting the process of sanding and polishing the clearcoat at the end. Be consistent so that when you scale up to the whole roof, your process is able to be replicated and you don’t end up with the hazing issues I initially had.
• Replace the glass trim. It is infinitely easier to do this job with the glass rubber seals removed. Hence, budget new trim into the project. I’m sure glad I did!
• Keep a "wet edge". When doing the clearcoat. Make sure that, on your heavy coats, you do 50-75% overlapping passes of a wet application; wet almost to the point of creating drips but not really. You want the clear to come out smooth and wet. If you do end up with some drips, you can adjust a bit during the sanding stage.


(to be continued as I think of more)

[GiorgioE82]

Questions?

[DrFerry]

THANK YOU very much for taking the time to write this DIY, adding to the community's body of knowledge, and chronicle your lessons learned. There will be others who come after who need this knowledge. Well done.
.

[0-60Motorsports]

Thank you for posting this! This will be an excellent guide to redoing my OE CSL roof clear coat. Cheers

[GiorgioE82]

Adding some answers to questions as I get them:

1. "Did you have to access the antenna sharkfin cover from inside the headliner to remove?"
No. The cover is just sticky taped to the roof. Use a trim tool to carefully pry it off by going underneath and around the bottom side of the cover and it pops right out.
You're then left with the "exposed" antenna which I just covered in masking tape before proceeding. No need to go into the headliner to do anything.

2. What's the part number for the roof clips?
Reading through other posts I saw people were having issues getting the correct roof clips for the roof rail/trim. I think it has to do with the facr that there are different clips for the regular steel roof vs the carbon roof. In any case the correct part number is 51138045321. They come in a strip of 7 clips for around $25-30. I've added a pic of what they look like (what remained of the 7 after I used a few, anyway) . Also, these have double sided tape on the bottom to stick to the roof. When I applied mine they didn't stick well so perhaps using a bit of heat or additional pressure will help. In my case I just put a small dab of black silicone on the bottom and pressed in place. Seems to have done the job. 👍

(to be continued)

[e46e92love]

This is crazy…..but well done. I took a slightly different path, I chose luck: the luck of the windshield guy AT THE DEALERSHIP damaging the roof while installing new windshield hence getting this all done for free by the dealership.

Voila: brand new looking roof on 10 year old, 90k mile car with zero effort😂😂.

Well done sir. Well done.

Cheers,
e46e92