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Old 24-01-2020, 09:19   #76
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

sorry, this thread is toooo loooong for my time ;-)

My biggest windows on my catamaran are about 4x4.5ft.
I asked my sealant producer and he suggested to split the tasks, a strong glue to hold the Polycarbonate to the frame, DO NOT UNDERSIZE, this has to take up bending in the glue as the plycarbonate expands and contracts, mine is 12mm thick, the glue.
Now that still leaves the gab around the windows, for this a very soft material was choosen, again, the gap is 12mm to take up 5mm movement. In the corner is a foam string so the polyurethane glues do not touch each other. 10 years on this cat, another 12 years on my last one, do not underestimate, give allowance to work, 100% elasticity is nothing if your gap is only 3mm!
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Old 24-01-2020, 09:40   #77
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Quote:
Originally Posted by blubaju View Post
sorry, this thread is toooo loooong for my time ;-)

My biggest windows on my catamaran are about 4x4.5ft.
I asked my sealant producer and he suggested to split the tasks, a strong glue to hold the Polycarbonate to the frame, DO NOT UNDERSIZE, this has to take up bending in the glue as the plycarbonate expands and contracts, mine is 12mm thick, the glue.
Now that still leaves the gab around the windows, for this a very soft material was choosen, again, the gap is 12mm to take up 5mm movement. In the corner is a foam string so the polyurethane glues do not touch each other. 10 years on this cat, another 12 years on my last one, do not underestimate, give allowance to work, 100% elasticity is nothing if your gap is only 3mm!
Wish I had an expert to go to to tell me what to do like that. LOL that would have save me countless hours of experimentation.

I basically adopted the exact same approach after all the failures. VHB tape is holding the poly carbonate just fine. It's the thickest they make and the widest as well. After failures with that product as well, I did call 3 AM on that. And they made the recommendation for the very thick and very wide VHB tape. And it works. I broke it up as well putting sealant is around the crack. Dow 795.

Glad to hear this is the preferred approach that the professionals use. I couldn’t think of any other way to do it.
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Old 24-01-2020, 09:51   #78
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

I used GE SilPruf to install 3í x 5í curved polycarbonate windows on a large catamaran. It was recommended by a guy who had a business installing all kinds of Lexan and Plexiglass panels on boats and airplanes. Used to be difficult to buy, but now available on Amazon. Here is the spec sheet link:
https://www.siliconeforbuilding.com/...836001c1/.aspx

GE also has several other similar caulk/sealant/adhesive products. Installing large polycarbonate panels on boats is not for the faint of heart. Also,correct preparation per the instructions is very important.

The windows I installed performed great, much better than whatever was used by the manufacturer of the vesseló after 5 years those windows leaked and cracked because the screw holes were too small.

Good luck with your project.
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Old 24-01-2020, 10:27   #79
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Sorry, I tried reading though all the pages, but not sure if it helps. I did the same approach for my boat with the VHP tape (1/4" wide), but used Dow 791 instead. It has held up great with no leaks. The tape was only used to hold the poly in place till the 791 cured, which was only about a day in 70 degrees.

After it cured, I cleaned up the edges and applied more 791 to any areas that didn't look good.

Maybe you can try and overcoat with 791 on the top to see what happens.
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Old 24-01-2020, 13:07   #80
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Here is an article I came across with good information.
https://www.sailmagazine.com/diy/rep...xed-portlights
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Old 24-01-2020, 17:36   #81
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

GE Silpruf SCS 2700 ������������

Polycarbonate

No fasteners after cure (7 days).

Super easy.
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Old 24-01-2020, 18:10   #82
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Iv'e used Silpruf SCS 2000 msny times over many years without an issue. Unfortunatly I tried Dow 795 once, i will never use it again. While tempered glass is a far superior product to either acrylic or polycarbonate in many ways it is a little over twice the weight so not practical if the windows are large and sailing performance is at all important to you.
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Old 24-01-2020, 18:47   #83
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

A lot of good information here now. Thank you for the extra info.
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Old 24-01-2020, 21:23   #84
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Geesh, 80+ posts on a simple technical issue.... maybe somebody could list all the options that work, in one single post?

Quote:
Originally Posted by Franziska View Post
Sikaflex 295uv is a sealant/glue specifically formulated to glue artificial glass panels on boats.
It has UV resistance built in.
It needs a special primer and a special black undercoat when used properly. Both are also available from Sika.
They also have very precise specifications and informations as to how to use the product for best results.
I reglued windows on my boat as well on clients boats. Works great if done properly.

Sika 296 is the correct sealant and glue for real glass windows in marine applications. It also needs the correct primer and workflow.

https://usa.sika.com

They have Pdf files with instructions on their site for both.
I use the same products as above, yes, expensive; yes, it works; yes, a thick bead is important; yes, there needs to be sufficient room on the edges to allow for expansion; yes if screws/bolts are used, then larger holes are important to allow for such expansion; yes, a rebate (with a small router and rounded domehead) in the acrylic will help; yes.....silicone products have its place; and yes, I am trying to avoid products with silicone, see also post #45.
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Old 24-01-2020, 21:56   #85
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Look into GE silpruf sc2000 - also used in commercial glazing

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Old 28-01-2020, 21:57   #86
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

It looks like a three-sided joint!

Dow 795 technical manual warns against three-sided joints, and recommends non-stick tape to cover the third side. Think of a U. You want the sealant to stick to the sides, but not the bottom. If it sticks to the bottom, the flexibility of the joint is severely compromised.

Dow recommends 1/4" minimum thickness. So that would be a good target, even if you couldn't get all the way there.

Dow also recommends a maximum of 3-to-1 width-to-thickness ratio. At 1/4" thick, that is only 3/4" wide. At 1/8" it is 3/8" wide. Counter-intuitively, wider joints are not necessarily stronger. Similarly, they recommend an hour-glass shape joint -- curved-in not only on the top, but the bottom as well. Another case of less adhesive being stronger.

The Dow technical manual is long and dense, but full of critical information.
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Old 29-01-2020, 02:25   #87
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Quote:
Originally Posted by daydreamer View Post
It looks like a three-sided joint!

Dow 795 technical manual warns against three-sided joints, and recommends non-stick tape to cover the third side. Think of a U. You want the sealant to stick to the sides, but not the bottom. If it sticks to the bottom, the flexibility of the joint is severely compromised.

Dow recommends 1/4" minimum thickness. So that would be a good target, even if you couldn't get all the way there.

Dow also recommends a maximum of 3-to-1 width-to-thickness ratio. At 1/4" thick, that is only 3/4" wide. At 1/8" it is 3/8" wide. Counter-intuitively, wider joints are not necessarily stronger. Similarly, they recommend an hour-glass shape joint -- curved-in not only on the top, but the bottom as well. Another case of less adhesive being stronger.

The Dow technical manual is long and dense, but full of critical information.
Now that was a good read!!

I wish I had seen that before I did this work.

I'm using Dow 795 all wrong! It's a structural glazing silicone when I needed a waterproofing sealant! It's not a 3 sided joint but I do have several of those to do where these windows touch each other.

The interesting part is I could have used only Dow 795 if I had done it as they show in this manual.

With the VHB tape doing the structural work, all I needed was some waterproofing sealant. Or I could have skipped the VHB tape and floated these windows to 1/4" off the surface and used only Dow 795.

Thanks so much. This wins best post. Took the mystery out of it all.
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Old 29-01-2020, 18:19   #88
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

The 3-to-1 ratio means the U is at most three times as tall as it is wide.

And I guess the non-stick tape is often called bond breaker tape. Or, if the U is too deep, you can use backer rod as the bond breaker, which will also fill the gap to keep you within 3-to-1.
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