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Jul 29, 2018, 11:32 AM
S55
S55
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Help!

Fiber glassing foam - what did I do wrong?


I intend to fiber glass a foam wing and started to experiment with some white foam panels first.

Following advice found on several videos, I mixed epoxy with denatured alcohol (50/50) and applied on foam first, then applied the cloth and brushed on top of it again. Then soaked with toilet paper. People said this way you remove excess resin (weight savings) and stiffness does not suffer because more resin doesn't improve stiffness; it's the cloth that matters for strength.
One panel was left uncovered, the other was covered with plastic film and I placed a smooth heavy slab on top to prevent warping (I didn't know whether this would be necessary).

They were left overnight (aprox 10hours) and this morning the cloth seemed to be dry, but it didn't feel hard. When I bent the panels to asses stifness, they felt soft and mushy.

This doesn't seem right. I either did something wrong or more drying time is needed. Please advise.

Thank you,
S55
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Jul 29, 2018, 01:43 PM
A.K.A sir Crashalot
methuso's Avatar
Ive read some about different denatured alcohol containing different amount of water, but i dont know how to check it and it dont say on the bottle. And water is bad for epoxy as i understand it.
Please,someone, educate me/us on this
Jul 29, 2018, 02:46 PM
If it flies, I can crash it.
rocketsled666's Avatar
You don't say what kind of epoxy you used. Some epoxies can take 24 hours to fully set. Mixed with Alcohol, that time can increase.

You can accelerate epoxy curing with heat. Hit it with the air from a blow dryer for 5 minutes, let it cool down, and see if it gets any harder.

Small amounts of water (<1%) can accelerate curing and increase flexibility of the cured epoxy. Larger amounts can significantly weaken the cured epoxy.
Jul 29, 2018, 03:04 PM
S55
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I used 20 min finishing epoxy. The video mentioning using alcohol to thin the epoxy said you can even sand after overnight curing.
Jul 29, 2018, 04:07 PM
If it flies, I can crash it.
rocketsled666's Avatar
Hmmm. Not sure what makes an epoxy a "finishing epoxy" is. But 20 minute epoxy should have cured out in about 12 hours.

I use 90% drugstore IPA to dilute my epoxies and I haven't had the problem you're describing. 90% IPA is 10% water so I'm not sure water is the issue with yours. But I tend to work in small batches with 5 minute epoxies that cure much faster.

Heat will accelerate the cure, and it'll drive out any remaining water. Try heating with a blow dryer to see if that'll harden it up. You could also try diluting just some hardener in alcohol and painting a thin coat on the epoxy that's already partially cured to see if a little extra hardener will get it to stiffen up. Though, this will leave a sticky residue you'll need to scrub off with more alcohol after everything is cured.
Jul 29, 2018, 04:35 PM
S55
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In several videos people mention using finishing epoxy which is pretty fluid to begin with. Mine says it can be sanded in 12 hours, full strength in 24.

Meanwhile surface got harder, so I guess more curing time was needed.

Another thing is warping. Both panels are slightly warped, even the one that cured under a slab.
Jul 29, 2018, 06:02 PM
Duane, LSF IV
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50/50 is way too much alcohol. That much will adversely affect the epoxy curing. Extra time and some heat may help, but it won't be full strength.. I think you don't want to add more than 1/10th. You should use a resin intended for lamination. Finishing resins can work, but a proper laminating resin isn't much more expensive. Strength is a combination of fiber and resin. Too much or not enough of either yields a weaker part.

As for warping, that's often caused by unbalanced fabric. If the fabric has a different numbers of threads in each direction (along its length, and across), or the threads are significantly different in weight or strength, the resin will shrink at different amounts in each direction, causing the warp. To fix it, make sure the fabric is oriented the same way for top and bottom surfaces. It's very easy to accidental reverse it. Also, keep the part in the vacuum bag or release film as long as you can.
Jul 29, 2018, 06:22 PM
S55
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Will try with less alcohol. I don't think the epoxy itself is the culprit. People report using even 5 min epoxy for faster drying time.
Jul 30, 2018, 10:16 AM
The Junk Man
As said, 50/50 turns epoxy to rubber. Use no more than 10 percent if at all.

I have never seen an epoxy manufacturer that said diluting with alcohol made their product better. if you want thinner epoxy, use a thinner epoxy. There is a huge difference in viscosity amongst epoxy types. WEST for example is 1000 CPS while my favorite, Adtech 820 is half that at 500 CPS.

Tom
Jul 30, 2018, 02:25 PM
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So you are not suppose to add thinners to epoxy if you need structural strenght. Ten % thinner reduces the composites strenght significantly.

You didnt mention if you used any balsa or ply over the foam. This adds a significant amout of strength to the wing.

These videos that show people adding thinning agents to epoxy resin are bogus. Yes it looks and sounds like a good idea, but ir lowers the tinsel strength of the finished composite. If may take 2 weeks for the alcohol to gas off and who knows how long for the water.

If the epoxy you are using is not a lamninating epoxy resin e.g. too thick, your using the wrong product or its too cold to be applying it. Best temp is 80 to 85 degrees F.

The other problem is you might end up with a warped wing afer the alcohol gases off as it displaces the epoxy molecules. Another problem that happens when you add a thinning agent to epoxy is it cools down the resin.

After you've mix a batch it heats up . This heat is needed to form the epoxy molecule chain . If you add a thinning agent it cools this process. The more you add the larger the cooling effect.

Most epoxy TDS instructions will say not to use a thinning agent if you need structural integrity.

Do not believe any video of someone adding a thinning agent to a batch of mixed resin, especially rubbing alcohol. The alcohol will gas off but the water may stay entraped in the the composite that you just made, making a weak useless part.

If you need a thinner epoxy resin buy an infusion resin. They have awesome wet out on fiberglass due to their very low viscosity.

The only suggestion i can make is to build a box large enough for the wing to fit in then put a 40 watt bulb inside the box with a small fan to circulate the warm air for a couple of days too try to gas off the alcohol and water. You dont want to get it too hot either, 150 degrees F would work. An autoclave would be perfect.
Jul 30, 2018, 02:53 PM
S55
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Tom, thanks for the recommendation. I looked at Adtech 820 and tend to give it a try, but in the same time I started to doubt that glassing the foam is the optimal solution for me.
Maybe it helps if I describe what I am after. I have the foam wings already cut from white foam. 66 inch wingspan, 9 inch at the root and 4 inch at the tip. Total weight of the wing set is 2.75 oz. It's for the Andover. My main goal is light weight as I want to fly this indoor.

After experimenting with few foam panels, aside from the issues already mentioned, I see that the total weight of the panel increases roughly 3 times after covering with cloth both sides. That's huge, it will totally ruin my chance of flying indoor. I'm using 0.75 oz cloth (the lightest I found at the local hobby store).

I tried another experiment. I cut some 1/4 inch wide strips of 1/64 birch plywood and epoxied them along a foam panel, one on top, one on the bottom. The stiffness after curing is unbelievable and additional weight is minimal. This would definitely suit my purpose. The unsolved problem is now the thin trailing edge which can easily be damaged inadvertently when placing the model in the trunk for example. I thought about gluing some 1/32 balsa strips underneath , but together with the glue, this will weigh a lot.

So I'm not sure what's a good way to solve this. Maybe cut another set of wings from blue or pink foam which is stiffer than the white one. It's heavier too, but it may not need all the additional reinforcing so in the end it's a lighter solution.
Jul 30, 2018, 02:58 PM
S55
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Roguedog, thanks for your input as well. I work in the garage and these days it's around 80 inside.
Maybe the post that I was writing while you posted yours will help you help me with some advice.
Jul 30, 2018, 04:37 PM
The Junk Man
I've really never heard indoor people talk about epoxy coverings. And you used 0.75 ounce cloth, which is heavy enough for quite large models. i used it on my P47 and really wished I had used 0.53 which is easily available from Steve Thayer at Thayercraft.

Here's the link: https://www.thayercraft.com/

And note all his prices include shipping. And he ships it rolled. Never buy folded fiberglass cloth.

Tom
Jul 30, 2018, 06:25 PM
S55
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I haven't flown indoor in the past and don't know any indoor hobbyist, it was just my thought that fiber glassing would be a light way of straightening the wings.
Searching the forums since starting this thread it seems people using fiber glassing are not concerned about weight to the degree I am.
Jul 30, 2018, 09:52 PM
Scott
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I agree with Rogue, thinning the resin is a bad idea, and also using only fabric and resin to cover a foam wing will not give you the results you are after. Get another set of wing cores cut from light (1 pound) foam, make sure you get the shucks with them. Sheet the wings with a quality laminating resin and 1/32" balsa. This will give you the strongest and lightest wings. If you have never sheeted a foam wing with balsa and laminating resin let us know, many of us will be happy to point you in the right direction.

Scott


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