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Old 04-04-2021, 21:33   #1
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Window Rebedding Compound to use?

To anyone with Window experience.

Hi,

This windows leaks. The boat is 18 years old and this side sits in the hot sun.

Today, I separated the window and dug out the old material. Anybody know what this gasket is? Is it butyl?
What do I use to re-bed the window? Silicone? Polyurethane? I am thinking of going to the automotive store and getting a tube of RTV.

18 years later, I assume there is some better materials out there.
What would you use?

Thanks,
Mark
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Old 04-04-2021, 22:26   #2
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Re: Window Rebedding Compound to use?

Lots of recent threads on this. Dow 795 silicone probably best. Polyurethane gets chalky. You can do a surprisingly good job though with a squeeze tube of black RTV silicone (permatex or similar) if you prep and mask it correctly then peel the tape right away. The correct procedure is to first adhere the window to the flange (mask and tool the inside bead) then mask and fill the "beauty" bead on the outside.
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Old 04-04-2021, 23:37   #3
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Re: Window Rebedding Compound to use?

We swear by Butyl - you can get different varieties, including a runny goop at the hardware store in a caulk tube (look for gutter sealant, works OK but is a horrible mess), or use butyl tape stuff ,also from a hardware store (it's used for sealing all sorts of things in construction), or our favorite, butyl from Compass Marine.

Butyl is also great when you need to disassemble (ie remove) a part, like a window or thru deck fitting. It does not go hard like a polyurethane or polysulfide.

Butyl is easy to apply, easy to clean up, and never goes hard.

We used it when we rebedded all our deck fittings, as well as when we installed new portlights. It just works great. No leaks after 8 years.

The stuff from Compass Marine has a really good stiffness, which is why we like it. It is soft enough to squeeze into all the crevices, but stiff enough that it does not end up everywhere, like the gutter sealant.

We used 795 on rebedding acrylic hatches. In this case, the bedding material has to hold the parts together - in other words it is an adhesive as well as a gasket forming material. Butyl would not work here. But it is hard to work with - super messy, so be really careful with your masking, and surface prep is critical.

If you just need a gasket (for windows that are held in place by screws or bolts) then the sealing material does not need to be adhesive - it just needs to seal. That's where butyl shines.

Good luck!
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Old 05-04-2021, 03:15   #4
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Re: Window Rebedding Compound to use?

Here's the link to Chotu's recent Window Saga thread. Lots of useful discussion for you, Mark. Good luck!

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Window Saga - New Thread Small Question:
https://www.cruisersforum.com/forums...on-246862.html
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Old 05-04-2021, 04:57   #5
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Re: Window Rebedding Compound to use?

Thanks Toaster.



"Butyl is also great when you need to disassemble (ie remove) a part, like a window or thru deck fitting. It does not go hard like a polyurethane or polysulfide."


This window doesn't have any mechanical fastening, buy it's not like a hatch where is it is getting stressed. It just needs to sit there and look pretty. I think the butyl is the best choice. It needs to seal it and remain flexible. What I took out was easy to remove.



Mark
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Old 05-04-2021, 17:13   #6
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Re: Window Rebedding Compound to use?

Quote:
Originally Posted by Windseeker View Post
Thanks Toaster.



"Butyl is also great when you need to disassemble (ie remove) a part, like a window or thru deck fitting. It does not go hard like a polyurethane or polysulfide."


This window doesn't have any mechanical fastening, buy it's not like a hatch where is it is getting stressed. It just needs to sit there and look pretty. I think the butyl is the best choice. It needs to seal it and remain flexible. What I took out was easy to remove.



Mark
Butyl usually needs some compression such as screws, through bolts or a flange. If these are “plastic“ windows , Dow silicon hard to beat -it’s what they use to install high rise windows
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Old 05-04-2021, 17:56   #7
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Re: Window Rebedding Compound to use?

These appear to be just stuck on there. There is nothing holding them in place except the adhesive/sealant. A great case for dow 795.

Butyl would not be a good choice. Because it will not hold these windows in place. That’s not what it’s for. It is more of a soft gasketing type material. The Dow 795, if applied thickly underneath the windows, will give as they expand and contract.

After all the hell I have went through over the years with my windows, due to their size, I can safely say the Dow 795 will work for you.

What you need to do, is leave a nice thick bead. Float those windows off the surface by a quarter inch or more. Then they will last forever.

Because they are small, you won’t have the thermal expansion issues that I had.
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Old 05-04-2021, 18:06   #8
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Re: Window Rebedding Compound to use?

The information you're getting is correct:

Butyl - must be clamped by screws or bolts. It has no adhesive property of its own. Fantastic for the right kind of port.

Dow 795 - Wonderful. Used to hold glazing to skyscraper walls with no hardware. Easier than most caulks to apply as long as you tape the edges. A finger smooths it beautifully.

As mentioned it must be 1/4" thick or the different expansion rates of the acrylic and fiberglass in the sun will cause it to rip loose quickly. The 1/4" thickness gives it enough material to stretch. One idea I used successfully was to make a tiny rim of 1/4" x 1/4" black foam weather stripping tape to support the acrylic. Then fill with 795 behind it. The foam will support the acrylic while the 795 cures while keeping the caulk from oozing out. Makes a nice neat look. Dow 795 tubes have an expiration date. Never use a tube that's past that date.
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Old 05-04-2021, 18:20   #9
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Re: Window Rebedding Compound to use?

My neighbor at the marina did similar windows with butyl. If you want a revolting, permanently oozing mess, that leaves marks on the clothes of everyone who brushes against the windows, with the windows ultimately falling out, that's the way to go.

Or you could use a proper adhesive caulk.
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Old 05-04-2021, 21:50   #10
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Re: Window Rebedding Compound to use?

For that window, black RTV is fine and will last for years in the hot sun, it is a repair, you are not doing all of them (yet). Mask everything, they also sell 3m fine line tape (1/4" green) at the automotive store, it makes curves easy. Wear gloves and be careful not to make a mess of the silicone everywhere. Peel the tape right away and you are good to go.
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Old 06-04-2021, 07:18   #11
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Re: Window Rebedding Compound to use?

Thank you so much for the information!

TO ALL: Yes. These windows are freely floating in their "frames" (if that is what I can call them). It is space that has been molded into the fiberglass. The is no mechanical fastening of the polycarbonate. The window sees no action like a hatch or port window. So, it needs some adhesion properties, but not a lot for bonding strength. It needs to be weather tight and super flexible. Whatever I took out was descent as it lasted 15 years and the other three windows do not leak . . . yet. It should be serviceable as these windows won't last forever.

Chotu - I Loved your H Channel Solution for the two windows. Very creative.
"Butyl would not be a good choice. Because it will not hold these windows in place. That’s not what it’s for. It is more of a soft gasketing type material. The Dow 795, if applied thickly underneath the windows, will give as they expand and contract." - Thanks for this insight.

CarlF - "tiny rim of 1/4" x 1/4" black foam weather stripping tape to support the acrylic" - Great Idea! I was thinking I would have to sit there and hold them in place until they cured.

Thumbs Up - fine line tape (1/4" green) - Another fantastic suggestion. I always wondered how you made curves like that.

Sounds like Dow 795 is somewhat of a consensus if applied with a thick 1/4" bead. RTV is also a possibility as well. I have used RTV on a thermostat housing, so I know it can take the heat. I would need to look at the UV performance of RTV.

You guys are great! Thanks so much for the ideas.

Mark
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Old 06-04-2021, 08:03   #12
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Re: Window Rebedding Compound to use?

Wind, I just did a similar job on my moody. since I had some curvature to deal with, I ended up using some 3m VHB tape in conjunction with the Dow 795 to hold the window to the cabin while the 795 set up.




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Old 06-04-2021, 11:39   #13
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Re: Window Rebedding Compound to use?

Hi Marc,

That looks nice!

I feel like I'm about to ask a super remedial question.

Did you put the tape on the window add glue around it and then set it in place? I'm under the impression from CarlF post that the 1/4" foam tape was there as a shelf or ledge to rest the window piece on.

I don't think I want to tape the outside of the window to the boat to hold it as it might force the sealant out and not maintain a 1/4 bead.

I'm trying to picture how I would execute. It does not look difficult. Just a little planning....

Thanks,

Mark
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Old 07-04-2021, 04:09   #14
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Re: Window Rebedding Compound to use?

From my good experience, use 3M VHB tape, in full uninterrupted perimeter, other than join to hold it in place and as waterproof seal. Add Fixtech sealant for acrylic/polycarbonite to seal around 3M tape to keep water away from the tape only. Make sure to use isopropop alchohol cleaner to both surfaces as per 3M guidance, and abrade acrylic with 100 grit prior to cleaning. One way wipe. This will be much. Better than sika flex or Dow as adheasive. Use the 3M VHB as the adhesive to hold it in place and sealant as secondary water seal. Select VHB thickness as 3mm or so to suit differential movement between Acrylic and Fibreglass over expected temperature range, including direct sunlight. Think ness of VHB allows. Ertian movement in mm vs thickness of VHB. See 3M guidance. The thicker VHB the greater movement and temperature range. Ideally calculate the differential movement range, to select VHB thickness. Also cut larger acrylic panels into smaller sizes with movement joints between to allow for thermal expansion within range of VHB, sealant thickness.
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Old 07-04-2021, 04:25   #15
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Re: Window Rebedding Compound to use?

i did as Peg did, a continuous tape across the top and then sides and then bottom tape.


Being I had some curvature to the cabin. I felt the tape was my best option... since i was deleting the aluminum frames altogether. and getting rid of the 50 or so screw holes around the edge of the window. once the tape gripped I then then did the blue tape around the innner and outer edges, cut a narrow tip on the 795 and then squeezed it flat so I could inject the 795 into what little bit of gap there was. then finger smear everthing.



Be forewarned. you don't get a second chance to reposition the window. and extra pair of hands/eyes go a long way.




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