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Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

A forum for discussing boat or trailer repairs or modifications that you have made or are considering.

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Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby C Buchs » Mon Jun 11, 2018 1:21 pm

I sealed the front windows on my 2002 :macx: two years ago using 3M 4000UV. I scraped everything and cleaned everything with isopropyl alcohol. I thought I had it. Turns out I didn't. I went to the boat yesterday and it was raining hard and the windows were leaking BAD. I've got a trip planned for this weekend and a San Juan Islands trip planned in about a month, so I need to get this fixed. Hopefully right this time.

I've been watching Boat Works Today on YouTube (I think someone here recommended, thanks!) and I remembered seeing this episode called "How To Install Fixed Portlights". You can find it here: https://www.youtube.com/watch?v=MQyjxVUskd8 In the video he recommends using 3M VHB Heavy Duty closed cell acrylic foam tape and sealing the outside with Dow Corning 795 silicon. Has anyone used this method with their Mac? Do I need to screw them down with this method? Sorry about rehashing this topic again, but I couldn't find a thread that talked about using the tape.

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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby Rackman » Mon Jun 11, 2018 9:45 pm

I used butyl tape on almost everything...worked great!
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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby sailboatmike » Tue Jun 12, 2018 7:48 am

Ezra from Blue Water Yachts sent me a PDF on fitting windows, they recommend black silicon
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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby C Buchs » Tue Jun 12, 2018 10:19 am

Rackman: How long have your windows been sealed with butyl tape? Is your boat stored outside? Do you have a lot of traffic on deck?

I think the main problem is that these windows are regularly stepped on. We try not to, but I know it happens regularly. My boys, their friends and girlfriends are all adult sized now and they're always walking over the windows to get to the bow to jump off.

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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby Interim » Tue Jun 12, 2018 1:20 pm

We used 5200 and haven't had any problems.

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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby C Buchs » Tue Jun 12, 2018 1:38 pm

I was advised against 5200 because its not UV resistant. That said, I only got two years out of the 4000 that is UV resistant.

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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby kmclemore » Tue Jun 12, 2018 2:27 pm

The correct sealant, as used by MacGregor at the factory, is Dow Corning 795 Silicone Building Sealant - they just used that alone, bedding the windows in it. You can use 3M 5200, but it's a pain to remove if you ever have to later on - that being said, I did use it on my windows and they're still leak-free 11 years later.

Here is a link, below, to a write-up I did on replacing the windows... it may be helpful. (I will further note that it is CRITICAL to ensure the bonding surfaces are absolutely free of any remaining adhesives, grease, oils or dust - get everything down to clean, dry gel-coat before you install the new windows.)

viewtopic.php?p=74566#p74566
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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby Tomfoolery » Tue Jun 12, 2018 2:37 pm

kmclemore wrote:The correct sealant, as used by MacGregor at the factory, is Dow Corning 795 Silicone Building Sealant - they just used that alone, bedding the windows in it.

Hunter used that also. I have most of a case of it left after rebedding my Hunter ports, but it's so long expired now that I don't trust it. I just pulled it out because of this thread, in fact, and noticed the 'use-by' date being something like 5 years ago. :P
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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby Highlander » Tue Jun 12, 2018 2:42 pm

Tomfoolery wrote:
kmclemore wrote:The correct sealant, as used by MacGregor at the factory, is Dow Corning 795 Silicone Building Sealant - they just used that alone, bedding the windows in it.

Hunter used that also. I have most of a case of it left after rebedding my Hunter ports, but it's so long expired now that I don't trust it. I just pulled it out because of this thread, in fact, and noticed the 'use-by' date being something like 5 years ago. :P


I guess u could say that it,s a no longer tacky situation ! :D :D :D

J 8)
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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby sailboatmike » Tue Jun 12, 2018 10:25 pm

Dow Corning® 795 is a one-part, medium modulus, neutral cure silicone formulation. Dow Corning® 795 cures to a durable and flexible silicone rubber building joint seal, and can accommodate ± 50% joint movement in a properly designed joint.

± 50% Joint Width Movement
Medium Modulus Building Sealant for Expansion Joints
Structural and Non-Structural Glazing of Glass, Metals, and Plastics

Basic Use: Structural and non-structural glazing of glass, metal, and plastic; adhering stiffeners to building panels; weatherproofing applications, including perimeter sealing of doors and windows. Dow Corning® 795 is virtually unaffected by sunlight, rain, sleet, snow, ultraviolet radiation, atmospheric contamination, ozone, and temperature extremes. Joint performance does not change significantly with aging or exposure to weather.

NOTE: Should not be applied to building materials that bleed oils, plasticizers or solvents, or in totally confined spaces. Do not apply to frost-laden or wet surfaces, or to surfaces that are in direct contact with food. Check cartridge label and Specification Data Sheet for additional limitations.

Ref: http://www.crlaurence.com.au/crlapps/sh ... odelID=597
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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby misterww » Fri Jul 06, 2018 6:10 pm

I'm a bit late to the discussion but I replaced all my windows a couple years ago with BWY version from Ezra. They're a good fit but they leaked like a noodle strainer until I used clear Flexx-Seall. Yeah the stuff at Wal-Mert. Not a drop now. ... the best $15 bucks I've spent on the boat. Buy two of the ones with the nozzle head and you're good for all six windows.
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Re: Sealing Windows with 3M VHB Heavy Duty & Dow Corning 795

Postby Maraquita » Fri Jul 06, 2018 8:06 pm

I have just gotten the windows for my 26D, and the 795 Silicone sealant, from BWY. I would like to paint the gelcoat behind the plastic, but not cut out, a flat black. Would the RTV seal to a Krylon type flat black paint? Or does it need to stick to pure Gel-coat?
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