Fiberglass cloth lifting off plywood when removing mylar sheet

MProcket

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I'm in the process of adding a layer of fiberglass cloth to my plywood fins. When I remove the mylar sheets off the pressed fin, sometimes some of the fiberglass cloth gets stuck to the mylar and lifts off the plywood, resulting in half 'bubbles' at the edges as seen in the photos. The fin is throughly wetted with epoxy and it just occurs when trying to remove the mylar to trim the excess cloth in the leather cure stage. Do you have any advice or input in how to keep this from happening? The wood and cloth are wetted at each of these places, but it just seems like the localized adhesion of the cloth to mylar is greater than that of cloth to plywood....?

Thanks for your help!

My general process is as follows:
Wet one side of fin, add fiberglass cloth, add additional epoxy until transparent and smooth it out. Cover with sheet of mylar film.
Turn over, repeat with other side. Once both sides are done and covered with mylar, use squeegee to remove air pockets and excess resin from each side. Picture attached, but now I've started having excess cloth surrounding the entire fin but is similar to photo.
Compress under a stack of books until leather stage. Remove mylar sheets, and trim fiberglass using a scalpel.



IMG_6544.jpg IMG_6543.jpg
 

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JohnCoker

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Interesting; I haven't seen that. Maybe leave the Mylar on until full cure and just trim through it at the leather stage?

Alternately, you could try cutting into the film from the middle of one side and try to peel it it from the middle outwards. (I'm trying to think of some way to reduce the peel force at the edges.)
 

MProcket

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Interesting; I haven't seen that. Maybe leave the Mylar on until full cure and just trim through it at the leather stage?

Alternately, you could try cutting into the film from the middle of one side and try to peel it it from the middle outwards. (I'm trying to think of some way to reduce the peel force at the edges.)

Thanks John, I could try that to see if it helps. If I do that, I’d most likely make the cut in the middle of the TTW root edge as I don’t want to make scratches with the scalpel on the other edges.

And your guides have been very helpful!!
 

MProcket

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coat the mylar with PVA mold release.

I read that mold release can’t be used with wood unless the wood is sealed first, which I don’t want to do since I want the full chemical bond from the porous wood fibers to the surface of the fiberglass under the Mylar. I’d be concerned with the release agent mixing with the epoxy and causing it to weaken. Or would this not be the case?
 

rharshberger

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I read that mold release can’t be used with wood unless the wood is sealed first, which I don’t want to do since I want the full chemical bond from the porous wood fibers to the surface of the fiberglass under the Mylar. I’d be concerned with the release agent mixing with the epoxy and causing it to weaken. Or would this not be the case?
coat the mylar...not the wood
 

cls

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also, you might improve adhesion by scuffing the plywood, like 120 grit or maybe even coarser depending on the glass layup. yeah I know that hurts, because you spent weeks sanding the fins to mirror perfection...
 

rharshberger

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also, you might improve adhesion by scuffing the plywood, like 120 grit or maybe even coarser depending on the glass layup. yeah I know that hurts, because you spent weeks sanding the fins to mirror perfection...
Or better yet puncturing the ply with something very pointy.
 

MProcket

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also, you might improve adhesion by scuffing the plywood, like 120 grit or maybe even coarser depending on the glass layup. yeah I know that hurts, because you spent weeks sanding the fins to mirror perfection...

The plywood was rough, did 150 grit on both sides but no other sanding. I like using this Mylar method since it leaves a mirror smooth finish on the fin.
 

cls

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ok great, 150 ought to adhere nicely. next time wait a lilttle longer through the green phase?
 

rharshberger

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ok great, 150 ought to adhere nicely. next time wait a lilttle longer through the green phase?
also thin the epoxy slightly and pre-treat the wood first, wait for the epoxy to get to green phase then apply the fabric and more epoxy to the wood, the two epoxies will then link forming a chemical bond, instead of just a mechanical bond like would occur if the fabric and epoxy were applied on top of cured epoxy.
 

MProcket

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also thin the epoxy slightly and pre-treat the wood first, wait for the epoxy to get to green phase then apply the fabric and more epoxy to the wood, the two epoxies will then link forming a chemical bond, instead of just a mechanical bond like would occur if the fabric and epoxy were applied on top of cured epoxy.
I’ll try this too and see how it goes. I’ve been wanting to avoid doing it as a two-step method since that will approximately quadruple the time for each fin. Hopefully with all these methods I won’t have the problem anymore. 🤞
 

rharshberger

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I’ll try this too and see how it goes. I’ve been wanting to avoid doing it as a two-step method since that will approximately quadruple the time for each fin. Hopefully with all these methods I won’t have the problem anymore. 🤞
Welcome to composites, it gets easier for some things (as you gain experience) and much more complex for others.
 

reddrock

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I apply a coat of PVA, let it dry completely, then apply a second coat of PVA brushing at 90 degrees to the first coat and let that dry. I found it easier to peel off the post cure PVA when 2 coats are used. Of course PVA is water soluble, so you can always wash it off. Works well on making tubes too.
 

beeblebrox

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Don't use mylar. Use polyethylene sheet (Like ziploc bags) almost nothing will stick to it. Almost falls off epoxy... Do not try saran wrap, not the same it will stick...
 

rharshberger

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Don't use mylar. Use polyethylene sheet (Like ziploc bags) almost nothing will stick to it. Almost falls off epoxy... Do not try saran wrap, not the same it will stick...
Depends on what material it is not all mylars are created equal, the Grafix Duralar product (clear not metalized) is what I use (don't remember which thickness, I bought it at Hobby Lobby) when trying to achieve a smooth surface with layups mainly on fins when I am not planning to do tip to tip or other followup layers.
 

MProcket

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Depends on what material it is not all mylars are created equal, the Grafix Duralar product (clear not metalized) is what I use (don't remember which thickness, I bought it at Hobby Lobby) when trying to achieve a smooth surface with layups mainly on fins when I am not planning to do tip to tip or other followup layers.

These are the mylar sheets that I'm using: https://www.amazon.com/gp/product/B09M7LPG4N/

Like you said, I like the smooth surface they create after being pressed. I think my main next steps will be to use the release agent coating on the mylar, increase cure time before removing the mylar, and minimize lifting force on the edges when removing. If that doesn't work, I'll move to the two-step epoxy process and a different top layer (parchment, polyethylene, etc.).
 

jahall4

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These are the mylar sheets that I'm using: https://www.amazon.com/gp/product/B09M7LPG4N/

Like you said, I like the smooth surface they create after being pressed. I think my main next steps will be to use the release agent coating on the mylar, increase cure time before removing the mylar, and minimize lifting force on the edges when removing. If that doesn't work, I'll move to the two-step epoxy process and a different top layer (parchment, polyethylene, etc.).

IMO you are just introducing variables making the process more difficult then it has to be. Use a food vacuum, porous release paper, and breather and follow @JohnCoker videos. Here is my own based on his:
 

cas206

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Most boat books and videos that I've seen will coat the wood and then completely wet out the fiberglass on the bench. Then apply the wet fiberglass to wet wood. Smooth and apply more epoxy as needed. This keeps the cloth from sucking resin out of the wood while you are in the process of wetting it out.
 

KenECoyote

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Forgive me if this is obvious or too simplistic, but did you lift the mylar starting at the fin root and then when it gets near the outer edges, pull downward vs. up? Normally lifting of glued/stuck parts are due to pulling up from the edge.
 

MProcket

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Forgive me if this is obvious or too simplistic, but did you lift the mylar starting at the fin root and then when it gets near the outer edges, pull downward vs. up? Normally lifting of glued/stuck parts are due to pulling up from the edge.

I was trying to avoid lifting it straight upwards when removing the mylar, but wasn't paying as much attention to it as I should so might have been pulling more laterally up than laterally down. But definitely not too simplistic of a thing for me to not have done right.

True, but mylar wont stretch either

None of my fins were tapered, so the stretch on rounded edges doesn't matter for me as much. If I was doing that, I'd also use a pressure method like vacuum bagging that can apply non-orthogonal force to the fin surface rather than my old text/reference books and gravity.
 

jahall4

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... use a pressure method like vacuum bagging that can apply non-orthogonal force to the fin surface rather than my old text/reference books and gravity.

Vacuum is for flat surface too. If you had used it you would not have had the "lifting" problem you are posting about. Just that simple. Again, follow @JohnCoker's videos and you'll be golden.
 
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