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post #1 of 40 Old 03-18-2010 Thread Starter
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Seating New Windows

Any suggestions on the best product to seal acrylic windows to a fiberglass boat? I ordered four new windows and want to get it right the first time. The windows are held in only by the bonding material, no thru bolts.
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post #2 of 40 Old 03-19-2010
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I'll be doing the same thing in the spring, replacing all the windows on my catamaran with cast acrylic bonded to the fiberglass. They're too wide to thru-bolt, with the differences in thermal expansion.

I plan to do something like this guy did:

How to install boat windows and port lights

I'll be skipping the tape, I want a thick bed of sealant due to the window width (they are over 4' wide). Not sure how I'll keep the windows in place while the sealant cures, I may temporarily bolt it in place and then fill the bolt-holes with sealant after it cures.
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post #3 of 40 Old 03-19-2010
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Dow Corning 739. Others have suggested Dow Corning 795, but I think the 739 is a better product. I also called Dow Corning and verified this with them. THey suggest 739 over 795, but 795 will work.

I have found that MEK (Methyl Ethyl Ketone) is a good product for removing old 795/739. You can also use denatured alcohol I believe, but I think the MEK may be a better product for the old, stuck on stuff.

I also prepped the surface with Xylene before wiping down with MEK. MEK did not discolor our windows or fade them - but check first.

I want to warn you that it takes 7-14 days for the 739 to cure completely. It needs to be put on in 55 degree weather minimum. Also, the 739 has a very short shelf life so make sure you check the expiration date before applying.

If you own a Catalina, this information on how-to is in the next Mainsheet as I wrote it for the 400's.

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post #4 of 40 Old 03-19-2010
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BTW, I would like to know why others have suggested 795 or 791 over 739. THat was the product specifically reccomended by Dow COrning for the marine environment and that application.

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We've successfully used 795 on several projects, but never in an adhesive-only situation. We used it in the first place based on a recommendation from a highly regarded local boat repair outfit.

I'd certainly be willing to give 739 a try assuming it's readily available. Even the 795 we had to find a wholesaler - it's not in your average marine or DIY store.

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post #6 of 40 Old 03-19-2010
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BTW, I would like to know why others have suggested 795 or 791 over 739. THat was the product specifically reccomended by Dow COrning for the marine environment and that application.

Brian

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post #7 of 40 Old 03-19-2010
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Read a little more about the sealants

739 - this product is described as a Plastic Adhesive
791 - this product is described as "WeatherSeal"
795 - this product is described as "Structural Adhesion"

My search for data sheets led to this one for 795:

http://www3.dowcorning.com/DataFiles...c88021ffb4.pdf

and this one for 739:

http://www3.dowcorning.com/DataFiles...c880002241.pdf

Couldn't find one for 791, but "Black" isn't listed as a color so I'm not terribly interested in it anyways.

Sadly the data sheets describe different things - the one I'm most interested in, joint movement capability, is 50% on 795 and not mentioned in the 739 spec sheet. Of course the 739's elongation percent is 640%. What the difference is, I'm not sure...

The article I posted earlier also mentioned painting the inside of the plexi around the edges, to hide the sealant/tape and have the exterior look better. That of course means that the sealant will be clinging to the hull and to the paint, and then the paint is in turn clinging to the acrylic. Does anyone think that might pose a potential problem? From reading the 795 datasheet, I think it's a bad idea...

The 795 data sheet recommends priming with 1205 primer on acrylic and polycarbonate (a clear primer). Of course Dow is discontinuing 1205 production in 2010...

Last edited by LookingForCruiser; 03-20-2010 at 10:14 PM.
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post #8 of 40 Old 03-19-2010
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What I did and what most mfg do is to press it until it makes a uniform seal around it. You may want to mask it off to make it look good too, but be careful messing with it too much. Better to let it dry and cut it off. If you uniformly press against the "window-inner-edge" where the glass adheres and fill in all the gaps, then let it dry and cut off any extra, it looks good. THat is basically how Catalina does it.

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post #9 of 40 Old 03-19-2010 Thread Starter
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Any thoughts on the West Marine adhesives, like the 4200? The product description says the stuff is made to adhere acrylic to fiberglass.
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post #10 of 40 Old 03-19-2010
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4200 will be guaranteed to leak, it won't take the expansion/contraction
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