Sanding a CF layup

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BDB

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I did my first CF layup a couple weeks ago to cover the balsa skins on my AMW FiberMax. I didn’t need the CF for any structural reason, but I thought it might look cool. I wanted a smooth finish so I compressed the layup with Mylar. It came out terrible, but I thought I’d ask for some input before I threw in the towel. Thus far, I have sanded with 220 grit, and it sands remarkably easily. I could sand more, coat it with epoxy and add another CF layer, or I could just smear it with Bondo, smooth it and paint it. Is there any way to salvage it?
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Sand it down as much as possible without cutting into the CF, then coat with fresh epoxy. I have had the same issue with mylar and now just use perforated peel ply, it allows the air to escape along with excess epoxy its easy to sand then top coat with epoxy for a nice looking CF look.
 
Sand it down as much as possible without cutting into the CF, then coat with fresh epoxy. I have had the same issue with mylar and now just use perforated peel ply, it allows the air to escape along with excess epoxy its easy to sand then top coat with epoxy for a nice looking CF look.

+1 You'll see that you've hit the carbon when the sanding dust changes from white to gray. You'll probably also see the surface texture change. You might need to do two coats of extra epoxy but you probably won't need to add a new layer of CF.
 
Sand it down as much as possible without cutting into the CF, then coat with fresh epoxy. I have had the same issue with mylar and now just use perforated peel ply, it allows the air to escape along with excess epoxy its easy to sand then top coat with epoxy for a nice looking CF look.

This would have been my advice as well, however it looks like you have already sanded through the CF on the bottom pic, left fin. You can keep going but you are likely to just make it worse. I would either go all the way and redo if this is something you want to experiment with or just sand flat, spot putty and paint.

Yes CF does sand easily, which was a bit of a surprise to me as well. Just make sure you are wearing a breathing mask, or even better a full face one to protect our eyes as well.
 
I decided to try one more CF layer, for no other reason but to learn the technique. This time I used peel ply and it came out great!

I then brushed on some laminating epoxy to give a smooth finish, but now I have a new issue: it wont cure. After 2 weeks, it's still tacky.

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I'm using US Composites 635 with the medium hardener, and I'm sure that I weighed it out in the proper 3:1 ratio. Their website says that storing below freezing for extended periods of time is not good, so I assume the long New England winter did it in. Thank goodness this isn't a high performance, MD rocket.

I’m probably just going to order some new stuff and give it another coat. Any other suggestions?

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I decided to try one more CF layer, for no other reason but to learn the technique. This time I used peel ply and it came out great!

I then brushed on some laminating epoxy to give a smooth finish, but now I have a new issue: it wont cure. After 2 weeks, it's still tacky.

View attachment 341302

I'm using US Composites 635 with the medium hardener, and I'm sure that I weighed it out in the proper 3:1 ratio. Their website says that storing below freezing for extended periods of time is not good, so I assume the long New England winter did it in. Thank goodness this isn't a high performance, MD rocket.

I’m probably just going to order some new stuff and give it another coat. Any other suggestions?

Sent from my iPhone using Rocketry Forum

I have had the same issue with 635 and medium hardener staying tacky for long times. I got frustrated, and just lightly sanded a section once, and found that underneath was fine. It is as it had a very thin layer of uncured on top, so I went ahead and wet sanded the entire thing and got a good surface that was not tacky. Mine was stored around 70 degrees its entire existence, so it may be a different problem...I suspect high humidity during cure for my issue, but I really don;t know.

If you are going to overcoat, I would wipe off what you can with absolute ethanol before re-coating.
 
I have had the same issue with 635 and medium hardener staying tacky for long times. I got frustrated, and just lightly sanded a section once, and found that underneath was fine. It is as it had a very thin layer of uncured on top, so I went ahead and wet sanded the entire thing and got a good surface that was not tacky. Mine was stored around 70 degrees its entire existence, so it may be a different problem...I suspect high humidity during cure for my issue, but I really don;t know.

If you are going to overcoat, I would wipe off what you can with absolute ethanol before re-coating.

Thanks, Chris. I suspected that there might be a solid layer under the the thin layer of goo.

The chemist in me is wondering what happens to epoxy hardener when it gets cold. I think a little NMR might be in order.

(BTW...your Blue Devils looked pretty good against my Rams. Bagley was unstoppable. Good luck with Syracuse. It will be a battle of zones!)
 
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Thanks, Chris. I suspected that their might be a solid layer under the the thin layer of goo.

The chemist in me is wondering what happens to epoxy hardener when it gets cold. I think a little NMR might be in order.

(BTW...your Blue Devils looked pretty good against my Rams. Bagley was unstoppable. Good luck with Syracuse. It will be a battle of zones!)

If your ratio wasn’t correct you either end up with excess hardener or uncured resin. Just wipe it off with alcohol and you should be good. I would not even sand.
 
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(BTW...your Blue Devils looked pretty good against my Rams. Bagley was unstoppable. Good luck with Syracuse. It will be a battle of zones!)
snip

Bagley can do that, just needs consistency. Duke going zone (and it working) is borderline sacrilege. Duke was founded as a Methodist University, but I thought the true religion was Man-to-Man Defense.

Good luck with finishing up your rocket, I suspect it is pretty good under there...just need some sandpaper and a strong rotator cuff to uncover it.
 
If your ratio wasn’t correct you either end up with excess hardener or uncured resin. Just wipe it off with alcohol and you should be good. I would not even sand.

It worked! Thanks, Chris and Steve. The thin layer of goo just whipped off with isopropanol. I just spent the past hour sanding the fins smooth. I may give them one more touch up with a little epoxy, but I'm really happy with the results.
 
Thanks, Chris. I suspected that their might be a solid layer under the the thin layer of goo.

The chemist in me is wondering what happens to epoxy hardener when it gets cold. I think a little NMR might be in order.

(BTW...your Blue Devils looked pretty good against my Rams. Bagley was unstoppable. Good luck with Syracuse. It will be a battle of zones!)
As noted by others the ratio probably wasn't correct. That being said US Composites 635 doesn't like temps below 50 degrees F or it slows the cure way down or even wont cure for a while. Also US Composites 635 3:1 is by volume not weight, however weighing it is generally good enough, I cant remember what the actual by weight numbers are.
 
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As noted by others the ratio probably wasn't correct. That being said US Composites 635 doesn't like temps below 50 degrees F or it slows the cure way down or even wont cure for a while. Also US Composites 635 3:1 is by volume not weight, however weighing it is generally good enough, I cant remember what the actual by weight numbers are.

I weighed it using a balance that is accurate to 1 g. For these small batches, it's possible that the error in the measurement, combined with the volume-to-weight ratio difference, was significant. Next time, I'll use syringes when making small batches.
 
When this has happened to me I can assure you that the ratio was correct as I too use a 1 g resolution balance, and measure at least a 40 g batch. Under the thin tacky layer is also a nicely cured layer, so it is not that the epoxy ratio prevents proper cure, but something is happening at the atmospheric interface. Notably, I don't get this when I vacuum bag using the same epoxy. It is an easy workaround, so I don't worry about it anymore.
 
Is this what they call blushing? If so, lots of articles on it from various manufacturers.
 
Blushing is similar or even the same, usually its caused by cool temps and moisture, however in my experience US Composites 635 3:1 doesn't usually have issues with blushing unless the temps are under 50 degrees F.
 
After applying black rocketpoxy fillets, this thing is starting to look really good. I have a set of decals from Stickershock, and then I want to give it a gloss clear coat. Any suggestions on a clear coat that will make the CF and FG pop?

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After applying black rocketpoxy fillets, this thing is starting to look really good. I have a set of decals from Stickershock, and then I want to give it a gloss clear coat. Any suggestions on a clear coat that will make the CF and FG pop?

If you have access to a compressor and a spray gun, the easiest clear gloss is nitrocellulose lacquer. It will always harden, it can be wiped completely off if you really mess it up, it is easy to spray, it can be tinted with automotive 'candies' and trans tint dye, metallics can be added to it, it thins with acetone or lacquer thinner, and the list goes on. Each successive coat melts into the previous coats unlike 2 part urethanes. Rattle cans work, but not so well if you want a nice finish.

If you want some filling capability, catalyzed polyester is awesome stuff, but that is a fully professional, highly toxic finish. A non-pro would likely have a bad time with cat poly. You can pretty easily, and quickly achieve a finish like nice gelcoat with cat poly.
 
I forgot to add, if you wnt a nice finish, you need to fill the surface first, and get it 'dead flat'. You can do this by block sanding your existing surface, take out the highs spots as much as possible without damaging the cloth, sanding 'flat' into any resin on the surface of the lamination but not inoto the cloth. Then abrade any remaining low places with a flexible abrasive like a red Scotchbrite pad, then apply a coat of the laminating resin, let cure, sand flat with a block, repeat. When all the voids are gone, you can either polish the epoxy resin, or clear coat. The earlier in the process that things get 'flat' the less overall work in the end, and the better the result.

When things get really smooth in the finishing process, and you are polishing, the 3m films, 1000 and maybe 1500(wet), and 3m Trizact 3000 (also wet) on an orbital are a huge timesaver, and will yield as good a result as your skill allows. This is how fine automotive finishes are polished.
 
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