Yellowing carbon fiber roof repair

[Battle]

I recently took my car in for a cf roof repair. The shop sanded down the clear coat and re sprayed it. The roof is still yellow. My question is there a repair. I've read that repair is not possible because of re clear coating the roof. Thanks.

[GiorgioE82]

Originally Posted by Battle

I recently took my car in for a cf roof repair. The shop sanded down the clear coat and re sprayed it. The roof is still yellow. My question is there a repair. I've read that repair is not possible because of re clear coating the roof. Thanks.

Have them redo it. There is a proper way to do and it will not stay yellow.

[Battle]

Originally Posted by GiorgioE82

Have them redo it. There is a proper way to do and it will not stay yellow.

Is there anything I should tell them to do to get it right?

[InnerBlueSkies]

I know nothing about the repair, I don't even have a CF roof but from my years on this board, I know the repair is much more than a sand and a re-clear.

You may want to bring it to a shop that has done these types of repairs before. If you are paying next to nothing and want to be a Guinee pig.

https://www.m3post.com/forums/showthread.php?t=1406986

[Biginboca]

The part that is yellow is the gel coat, so they need to sand through the gel coat and remove all of it. They probably only did a light sanding just for adhesion of the new clear.

If you haven’t worked with CF before you wouldn’t know that the yellowed gel coat needs to come completely off.

[Battle]

Originally Posted by Biginboca

The part that is yellow is the gel coat, so they need to sand through the gel coat and remove all of it. They probably only did a light sanding just for adhesion of the new clear.

If you haven't worked with CF before you wouldn't know that the yellowed gel coat needs to come completely off.

How high of a probability is it to damage the roof while sanding the gel coat?

[Biginboca]

Originally Posted by Battle

How high of a probability is it to damage the roof while sanding the gel coat?

I’ve done it and didn’t find damaging the roof to be something to really worry about. You just watch for the sandpaper to start showing black instead of yellow, then you know you have gone through the gel coat. Once it’s black you need to stop sanding that area obviously you are now into the carbon layer.

If you get into the carbon too deep it’s not going to affect the roof structural integrity it can just affect the way the weave pattern looks once it’s cleared. But you would have to be looking really closely and looking for it, it wouldn’t be something I think anyone else would notice.

Some guys have had good luck just peeling the gel coat, there’s a few threads on here. If it’s damaged enough that you can peel it off then that’s a huge timesaver. The roof panel is a large panel to sand, especially if you are doing it by hand like I did. Either way to get rid of the yellow you want to get all that gel coat off and start fresh at the carbon.

[Battle]

Originally Posted by Biginboca

I've done it and didn't find damaging the roof to be something to really worry about. You just watch for the sandpaper to start showing black instead of yellow, then you know you have gone through the gel coat. Once it's black you need to stop sanding that area obviously you are now into the carbon layer.

If you get into the carbon too deep it's not going to affect the roof structural integrity it can just affect the way the weave pattern looks once it's cleared. But you would have to be looking really closely and looking for it, it wouldn't be something I think anyone else would notice.

Some guys have had good luck just peeling the gel coat, there's a few threads on here. If it's damaged enough that you can peel it off then that's a huge timesaver. The roof panel is a large panel to sand, especially if you are doing it by hand like I did. Either way to get rid of the yellow you want to get all that gel coat off and start fresh at the carbon.

Thank you so much for your reply. Here is what it looks like

[Battle]

Originally Posted by Battle

Thank you so much for your reply. Here is what it looks like

Also are there any other steps after sanding the gel? Do you re gel it and then clear coat?

[Biginboca]

Originally Posted by Battle

Also are there any other steps after sanding the gel? Do you re gel it and then clear coat?

I just clear coated it. It looked great at first and then after about a month the clear coat kind of shrunk into the weave, so what was a smooth glassy finish at first now has like the texture of the weave in its surface. So it kind of looks Orange peely from some angles but it’s not orange peel it’s the weave pattern.

So to avoid that you could do a gel coat first it’s thicker and would be more resistant to the weave texture showing though.

This is the same as most carbon parts look when painted. If you ever saw a painted carbon hood or trunk the weave texture often shows through the paint also.

Your roof looks great to me. I think knowing what I do now I would leave it alone you can always sand (or peel) it later down the line. It seems to me inevitable that these roofs will all need to be re coated at some point. The UV rays just take their toll over time.

[Battle]

Originally Posted by Biginboca

I just clear coated it. It looked great at first and then after about a month the clear coat kind of shrunk into the weave, so what was a smooth glassy finish at first now has like the texture of the weave in its surface. So it kind of looks Orange peely from some angles but it’s not orange peel it’s the weave pattern.

So to avoid that you could do a gel coat first it’s thicker and would be more resistant to the weave texture showing though.

This is the same as most carbon parts look when painted. If you ever saw a painted carbon hood or trunk the weave texture often shows through the paint also.

Your roof looks great to me. I think knowing what I do now I would leave it alone you can always sand (or peel) it later down the line. It seems to me inevitable that these roofs will all need to be re coated at some point. The UV rays just take their toll over time.

I have already paid the body for the job. I will take the car back on Monday for further repair. It is really yellow under the sun. I hope they can remove their clear coat and maybe peel the gel or sand it down and re gel it

[LakeSurfer]

So the yellowing is from the epoxy resin. Epoxy typically does not have any UV protection built into it, you would need to find UV additives. Polyester resin is typically UV resistant. Older boats and some new boats use polyester resin vs epoxy. Epoxy is a much better resin but more expensive. You can put epoxy over polyester but you cannot put polyester over epoxy. When epoxy resin is exposed to the sun it will start to yellow. It will look great for a short period of time but will soon degrade.

Gel coat is what you would see on a polyester boat, it's the color coat, typically white but you can tint it. and gel coat is polyester based and therefore not compatible with epoxy. I say this because no one producing high quality carbon fiber parts would use polyester resin. There really isn't an epoxy based gel coat. It would be safe to assume that BMW used epoxy resins and therefore no gel coat.

I think someone mentioned it but it you lay up carbon and use an automotive clear coat, it will shrink and look like it's sunk into the weave. This is how you can identify a real or non wrecked Ferrari F40. You can see the weave through the paint and if it's been wrecked and re sprayed, you would see flat snooty paint.

From what I see BMW used a few coats of epoxy resin over the carbon. They flat sanded it and either used a thin coat of automotive clear or didn't do anything.

You can sand all the yellowed coating off. You should wet sand it and keep it very wet and wipe the sanding residue off often. Wet sanding will give the appearance of a glossy finish you you'll need able to see the weave change if you sand to far. I would also dry it off from time to time and inspect the color of the sanding dust. If it's black, stop and if it's white or yellow you can keep going. You can actually sand a good amount of the carbon before you affect the weave but I wouldn't advise. It takes practice to know what to look for.

After it's sanded and if it were me, I would have the body shop hammer on maybe 5 or so coats of clear. No resin, just automotive clear. No need to wet sand and buff it yet, just shoot the clear and kick it out the door. Wait 2 months or for the clear to fully cure. Then come back into the shop and either wet sand and buff or wet sand l, soot a coat or two more of clear then sand and buff.

I think that covers it.

[Battle]

Originally Posted by LakeSurfer

So the yellowing is from the epoxy resin. Epoxy typically does not have any UV protection built into it, you would need to find UV additives. Polyester resin is typically UV resistant. Older boats and some new boats use polyester resin vs epoxy. Epoxy is a much better resin but more expensive. You can put epoxy over polyester but you cannot put polyester over epoxy. When epoxy resin is exposed to the sun it will start to yellow. It will look great for a short period of time but will soon degrade.

Gel coat is what you would see on a polyester boat, it's the color coat, typically white but you can tint it. and gel coat is polyester based and therefore not compatible with epoxy. I say this because no one producing high quality carbon fiber parts would use polyester resin. There really isn't an epoxy based gel coat. It would be safe to assume that BMW used epoxy resins and therefore no gel coat.

I think someone mentioned it but it you lay up carbon and use an automotive clear coat, it will shrink and look like it's sunk into the weave. This is how you can identify a real or non wrecked Ferrari F40. You can see the weave through the paint and if it's been wrecked and re sprayed, you would see flat snooty paint.

From what I see BMW used a few coats of epoxy resin over the carbon. They flat sanded it and either used a thin coat of automotive clear or didn't do anything.

You can sand all the yellowed coating off. You should wet sand it and keep it very wet and wipe the sanding residue off often. Wet sanding will give the appearance of a glossy finish you you'll need able to see the weave change if you sand to far. I would also dry it off from time to time and inspect the color of the sanding dust. If it's black, stop and if it's white or yellow you can keep going. You can actually sand a good amount of the carbon before you affect the weave but I wouldn't advise. It takes practice to know what to look for.

After it's sanded and if it were me, I would have the body shop hammer on maybe 5 or so coats of clear. No resin, just automotive clear. No need to wet sand and buff it yet, just shoot the clear and kick it out the door. Wait 2 months or for the clear to fully cure. Then come back into the shop and either wet sand and buff or wet sand l, soot a coat or two more of clear then sand and buff.

I think that covers it.

Thank you for your reply. Won't the clear coat get absorbed by the carbon in time?

[Biginboca]

Originally Posted by Battle

Thank you for your reply. Won't the clear coat get absorbed by the carbon in time?

I agree with what he has said. Wait a few months for the clear to shrink into the weave and then when you sand you will be smoothing the pattern out and getting back to the smooth glossy finish. (Think of the sanding as knocking off the micro “peaks” of the texture so they become flush with the “valleys” in the texture)

The mistake I made is I wet sanded and polished only a week after spraying my roof with 4 coats of clear coat. It hadn’t shrunk and hardened yet. So while it became glossy and beautiful at the time, as the clear shrunk and hardened over the month the texture became more pronounced. I should have waited a few months for it to fully harden and shrink, and then sanded and polished it.

I could sand and polish again but obviously each time you do you are removing a little more clear coat, and eventually you will go through it. So better to wait and only do it once.

[LakeSurfer]

Originally Posted by Battle

Thank you for your reply. Won't the clear coat get absorbed by the carbon in time?

The clear will sink into the carbon over time because most high quality auto paints take weeks to months to fully cure. So the work around is to spray several coats and leave it to fully cure and when it is you can sand it down just to even it out and then lay down the more clear. When you sand between clear coats the trick is to not sand all the way back to the carbon because if you do the clear will sink in again.

You can use high quality epoxy and then sand it down flat and then clear but it's more work and you could fave the same yellowing again.

Most body shops don't mess with carbon because of these reasons. It's a lot of work and it's easy to mess up. They want to shoot it one time and be done but that's pretty unrealistic.

I've been messing with forged carbon fiber for a while now well before Lamborghini started to use it it. When you make carbon parts you ha e a lot of waste and I hated that part of it so I cut it up into 1/4 inch chunks and laid it on top of epoxy resin. Then you can sand it as much as you want without worrying about burning through it and if you do, all you have to do is lay some more down in that spot and you would never know. But you still need to clear it multiple times as the clear soaks into the carbon the same way. I think I made a post about a couple parts I made for my M here somewhere. For these super car guys charging a premium for scrap materials that they would otherwise throw away makes my skin boil. It's non structural and way weaker than weave carbon. I tell you this because I want someone to do a forged carbon roof.

Something else to consider would be to do a satin clear coat on the carbon weave. I think satin weave looks so much more impressive than gloss too. However it can be more finicky to shoot it.

Sorry for my long posts. I use to work in an aviation composites shop and now I do personal projects in my garage. Currently building a foam and fiberglass flats fishing boat. So I'm kind of nerdy about this stuff.

I'm sure someone could disagree with me on some of this and I never consider myself a professional or a know it all but this is all based off my personal and professional experiences.

I'd be happy to help with any other questions.

[Battle]

Thanks for the reply guys. I will pass it on to the body Shop and hope they do it right. Will keep you posted

[akkando]

Originally Posted by LakeSurfer

The clear will sink into the carbon over time because most high quality auto paints take weeks to months to fully cure. So the work around is to spray several coats and leave it to fully cure and when it is you can sand it down just to even it out and then lay down the more clear. When you sand between clear coats the trick is to not sand all the way back to the carbon because if you do the clear will sink in again.

You can use high quality epoxy and then sand it down flat and then clear but it's more work and you could fave the same yellowing again.

Most body shops don't mess with carbon because of these reasons. It's a lot of work and it's easy to mess up. They want to shoot it one time and be done but that's pretty unrealistic.

I've been messing with forged carbon fiber for a while now well before Lamborghini started to use it it. When you make carbon parts you ha e a lot of waste and I hated that part of it so I cut it up into 1/4 inch chunks and laid it on top of epoxy resin. Then you can sand it as much as you want without worrying about burning through it and if you do, all you have to do is lay some more down in that spot and you would never know. But you still need to clear it multiple times as the clear soaks into the carbon the same way. I think I made a post about a couple parts I made for my M here somewhere. For these super car guys charging a premium for scrap materials that they would otherwise throw away makes my skin boil. It's non structural and way weaker than weave carbon. I tell you this because I want someone to do a forged carbon roof.

Something else to consider would be to do a satin clear coat on the carbon weave. I think satin weave looks so much more impressive than gloss too. However it can be more finicky to shoot it.

Sorry for my long posts. I use to work in an aviation composites shop and now I do personal projects in my garage. Currently building a foam and fiberglass flats fishing boat. So I'm kind of nerdy about this stuff.

I'm sure someone could disagree with me on some of this and I never consider myself a professional or a know it all but this is all based off my personal and professional experiences.

I'd be happy to help with any other questions.

What's the deal with the carbon fiber in the m2 with no clear on it? What process does that go through? BMW claims it to be a matte finish. It's textured though unlike the matte finish I've seen on other CF pieces. https://www.getbmwparts.com/oem-part...go-32302413480

[Battle]

Originally Posted by LakeSurfer

The clear will sink into the carbon over time because most high quality auto paints take weeks to months to fully cure. So the work around is to spray several coats and leave it to fully cure and when it is you can sand it down just to even it out and then lay down the more clear. When you sand between clear coats the trick is to not sand all the way back to the carbon because if you do the clear will sink in again.

You can use high quality epoxy and then sand it down flat and then clear but it's more work and you could fave the same yellowing again.

Most body shops don't mess with carbon because of these reasons. It's a lot of work and it's easy to mess up. They want to shoot it one time and be done but that's pretty unrealistic.

I've been messing with forged carbon fiber for a while now well before Lamborghini started to use it it. When you make carbon parts you ha e a lot of waste and I hated that part of it so I cut it up into 1/4 inch chunks and laid it on top of epoxy resin. Then you can sand it as much as you want without worrying about burning through it and if you do, all you have to do is lay some more down in that spot and you would never know. But you still need to clear it multiple times as the clear soaks into the carbon the same way. I think I made a post about a couple parts I made for my M here somewhere. For these super car guys charging a premium for scrap materials that they would otherwise throw away makes my skin boil. It's non structural and way weaker than weave carbon. I tell you this because I want someone to do a forged carbon roof.

Something else to consider would be to do a satin clear coat on the carbon weave. I think satin weave looks so much more impressive than gloss too. However it can be more finicky to shoot it.

Sorry for my long posts. I use to work in an aviation composites shop and now I do personal projects in my garage. Currently building a foam and fiberglass flats fishing boat. So I'm kind of nerdy about this stuff.

I'm sure someone could disagree with me on some of this and I never consider myself a professional or a know it all but this is all based off my personal and professional experiences.

I'd be happy to help with any other questions.

After removing the gel. What would be the best way to clean the roof? Alcohol. Acetone etc. I've seen it where the guy cleaned with alcohol and there was residue. Thanks

[LakeSurfer]

Originally Posted by akkando

What's the deal with the carbon fiber in the m2 with no clear on it? What process does that go through? BMW claims it to be a matte finish. It's textured though unlike the matte finish I've seen on other CF pieces. https://www.getbmwparts.com/oem-part...go-32302413480

Yeah this is another one I take issue with. I absolutely love the look of it but it's a less expensive to do than any other finish but a lot of times there is a charge for it. Not sure if BMW charges extra for it though.

Basically there are three main ways to do composites via baccum bagging.

1. Wet layup. This is applying resin to the cloth by hand, using rollers and various hand tools to impregnate the resin into the cloth. No major manufacture of any carbon fiber like BMW should be using this method and if they did they should be vacuum bagging it as well. However, it's still common practice in the different industries. It's cheap and inconsistent. But good enough for the backyard guy without vacuum pumps and accessories.

2. Resin infusion. Basically you layup dry cloth into a mold and vacuum bag it all down. There is a separate pot that is filled with resin and connected to your vacuum bag via hoses and a value. Open the valve and the vacuum will infuse the part with resin. When done properly and cured the part should come out with an almost perfectly glossy finish. This requires a lot of consumable materials every time you build a part. So it can be expensive and there is a lot of room for error, plus you need to monitor it and stop the flow of resin at some point or you'll end up introducing air that will ruin your part. High end boat builders use this.

3. Pre impregnated cloth. This is what I believe the parts you have are made of. I could be wrong but from what I know this would be the way to go. This is cloth has the resin pre impregnated into the cloth. Comes in rolls and is stored in freezers to prevent it from curing. It will never fully cure without high temperatures. The cloth is laid into a mold, probably on made out of aluminum. After that it is vacuum bagged and heat blankets or autoclaves are used to cure the part. When cured they pop out of the mold with the perfect amount of resin to cloth ratio and have a natural matte or flat appearance with, again if done right need no further prep if that is the desired result. But you will have a slight texture of the weave and if you wanted it to be gloss it would require sanding and prepping. You would have very little if any yellowing because there isn't enough resin exposed and even if it did it wouldn't be very noticeable if at all. They set it all up and put timers on it and walk away. Very little time is spent on creating parts this way. They can be trimmed and installed very quickly after cured. I can't say for sure if BMW did or diddnt put another layer of matte clear down but I don't think they did or would need to.

I made a couple pickgurads for a guitar I built using prepreg a few years ago and they still look great. If I scratch them, in my case, all I have to do is wet sand up to 2000 grit paper and it looks new again.

koenigsegg makes all there parts using prepreg and I believe so do all the other supercar manufactures, at least for the tubs.

[LakeSurfer]

Originally Posted by Battle

After removing the gel. What would be the best way to clean the roof? Alcohol. Acetone etc. I've seen it where the guy cleaned with alcohol and there was residue. Thanks

I think either work fine. Normally at home I use acetone and compressed air. The air blow any dust out if you have any pinholes. And then of course use a tack cloth before spraying. It's like cheese cloth with wax in it. The idea is that you lightly wipe the surface with it and it will pick up any remaining dust or debris.

However, if you want to be safe they do have products specifically made for cleaning surfaces before spraying anything.

[LakeSurfer]

I just found this video of a bmw carbon fiber roof. Looks like an E92 roof to me.

It also looks like they use prepreg material.

Another point is you can see the amount of waste at one point. Someone in the super car game got a huge raise when they came up with "forged" carbon fiber using waste material.

The roof is most likely out sourced to a composites company, then Bmw puts the roof on the car and clears everything at one time.

Sorry to the OP for hijacking but I hope some of this information helps out.

[Battle]

Originally Posted by LakeSurfer

I just found this video of a bmw carbon fiber roof. Looks like an E92 roof to me.

It also looks like they use prepreg material.

Another point is you can see the amount of waste at one point. Someone in the super car game got a huge raise when they came up with "forged" carbon fiber using waste material.

The roof is most likely out sourced to a composites company, then Bmw puts the roof on the car and clears everything at one time.

Sorry to the OP for hijacking but I hope some of this information helps out.

That a great video. Thank you