Bedding for windows

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Euphonyx

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Coachroof windows on my boat are perspex and bolted directly to the GRP coachroof sides. They are bedded with what I'd say is silicone and it's got dirty and the widows have started to leak. taking them off to rebed is one of the winter projects this year. what should I use as a seal? Sikaflex? Rubber gasket? Closed cell foam? Whats best in your opinion?
 
Worth establishing if the old bedding is indeed silicone, if it is you will need to remove every last trace of it before using something better. I have used Sikaflex but the consensus these days seems to favour butyl tape. Dont recommend CT1 which is an excellent sealant/adhesive but will make any further removal close to impossible, good but very permanent. If you do use sealant from a gun spend plenty of time doing a proper job of masking around the joints, remove tape before it sets and have a big bucket handy to dump the mucky tape in. Nitrile gloves are better than vinyl or latex.
 
I would go for butyl tape (or string).
I did this job on my boat some years ago. First time I just cleaned the surfaces and used Sikaflex. Next year I learned that I did it all wrong. You must have a primer on the glass. I also learned about butyl.
In the second attempt I sanded lightly the glass in the area of the seal and painted a black Sika primer. I used 3 sealants this time! Butyl string in the midle, black Sikaflex at the outer edge and white silicone between the glass and the interior vinyl liner. The purpose of the Sikaflex and the silicone was to prevent the forever soft butyl to squeeze out on hot day. This has worked for my boat.
A bonus: the even black primer is all I see from the outside. Any uneven width of the sealants is hidden.vindu1.JPG

The picture shows the glass with primer. Use masking tape.
 
eilerts said:
I would go for butyl tape (or string).
I did this job on my boat some years ago. First time I just cleaned the surfaces and used Sikaflex. Next year I learned that I did it all wrong. You must have a primer on the glass. I also learned about butyl.
In the second attempt I sanded lightly the glass in the area of the seal and painted a black Sika primer. I used 3 sealants this time! Butyl string in the midle, black Sikaflex at the outer edge and white silicone between the glass and the interior vinyl liner. The purpose of the Sikaflex and the silicone was to prevent the forever soft butyl to squeeze out on hot day. This has worked for my boat.
A bonus: the even black primer is all I see from the outside. Any uneven width of the sealants is hidden.View attachment 75358

The picture shows the glass with primer. Use masking tape.
Click to expand...
+1
Butyl tape. easy to use no mess and no water , put in a new hatch re-bedded 2 windows and front hatch all with butyl
not leaked , but make sure everything comes off before bedding
 
With aluminium frames against GRP, I took advice here http://www.ybw.com/forums/showthrea...on-a-few-hints-and-an-enquiry&highlight=scapa and used Scapa 3507 bedding tape, and have been so far very pleased with the result. It is adhesive on one side and requires compression to make a seal, and whether it would work with Perspex and in your situation I do not know, but it would avoid the stickiness of butyl tape and is worth thinking about.
 
geem said:
Butyl tape is great in cold climates but we have found that once in a hot climate the butyl goes soft and messy. We wont be using it again in the Caribbean.
Click to expand...

I know nothing about hot climate, but I do believe you are right. That is why I tried to confine this soft sticky mess with a Sikaflex seal on the outside and silicone on the inside - even in Norway.

I used a 3M product. The round string will take up more unevenness than than a flat tape.

20190119_194721.jpg DSCN6217.jpg

To the right in the right picture is the Sikaflex, then the butyl string
 
eilerts said:
I know nothing about hot climate, but I do believe you are right. That is why I tried to confine this soft sticky mess with a Sikaflex seal on the outside and silicone on the inside - even in Norway.

I used a 3M product. The round string will take up more unevenness than than a flat tape.

View attachment 75376 View attachment 75377

To the right in the right picture is the Sikaflex, then the butyl string
Click to expand...

We tried that on our steering pedestal but the butyl burst through the sikaflex making quite a mess. We really regret using it. It looked very promising but over time in hot weather it compresses and goes liquid
 
The standard for boat manufacturers and window glazing is Dow 795 silicone.

Sikaflex bond will be destroyed by UV within a few years (the UV goes through the glass, striking the bond area from the glass side) unless properly primed with a UV-proof primer. This is the leading cause of polyurethane seal failure, I've done both lab and field testing.

Butyl can make some acrylic craze. A roll of the dice. Same with polyurethane--I've seen this happen with 3M products.

Not just any silicone, ONLY a product rated for this. Dow 795 is VERY well proven.
 
geem said:
Butyl tape is great in cold climates but we have found that once in a hot climate the butyl goes soft and messy. We wont be using it again in the Caribbean.
Click to expand...

Did it take a while?

I got some of this for a few goosenecks but a bit wary now after what you said.

https://www.ebay.co.uk/itm/DOUBLE-S..._yAkRy8ZluUGn9bDRIBA:rk:4:pf:1&frcectupt=true

Put a bit a couple of inches in front of a fan heater last night and it seems OK, sticky but it was anyway.

DtZrRXo.png
 
thinwater said:
Sikaflex bond will be destroyed by UV within a few years (the UV goes through the glass, striking the bond area from the glass side) unless properly primed with a UV-proof primer. This is the leading cause of polyurethane seal failure, I've done both lab and field testing.
Click to expand...

I believe the OP was bedding Perspex/Acryllic, which is opaque to UV.

Personally, I am replacing the Perspex windows in my boat, and the major source of leaks is around the bolts. I am going for a glued-in solution, using Sikaflex 295 UV and the Sikaflex primer. No bolts.
 
thinwater said:
Not just any silicone, ONLY a product rated for this. Dow 795 is VERY well proven.
Click to expand...

I'm interested to try some of this now ... do you know where I can buy any online in the UK? I can find oodles of it in the USA, but not inthe UK so far, if you know where I can order a tube, I will order one and do an "initial bond" test on some Perspex to see how much "grab" it has compared to the Sikaflex 295
 
I have used neoprene foam for years on my wooden boats which if I am correct is actually open cell.
I have never had any leak, even when not all screws holes are good.

That, as well as butyl allows for thermal expansion of the windows which is especially important with perspex windows if they are a bit longer.

Plastic windows can stretch so much that they will eventually tear away any bond in a sealed window.

put an owl through the holes and push the screw through. During tightening it will seal around the screw.
 
thinwater said:
Not just any silicone, ONLY a product rated for this. Dow 795 is VERY well proven.
Click to expand...

It looks like this can be ignored in the EU, as it seems Dowsil 795 is a USA-only product. I have spoke to two major Dow distributors today, neither of them stock it, and cannot find it in Europe. In the USA, it's common, even on eBay!

Anyway, I shall write this one off as "not available here" and stick with the Sikaflex 295 UV
 
SvenH said:
I have used neoprene foam for years on my wooden boats which if I am correct is actually open cell.
I have never had any leak, even when not all screws holes are good.
.
Click to expand...

Interesting you should mention this, I recently fitted new windows to my wooden boat with the same method, as an experiment since I've not heard of anyone doing it before. My main reason was to avoid masking, since PC isn't very solvent resistant.

It leaked in the bottom corners, so I removed the outer foam and replaced with polysulphide, yet to see if it will keep the water out.
 
Euphonyx said:
Coachroof windows on my boat are perspex and bolted directly to the GRP coachroof sides. They are bedded with what I'd say is silicone and it's got dirty and the widows have started to leak. taking them off to rebed is one of the winter projects this year. what should I use as a seal? Sikaflex? Rubber gasket? Closed cell foam? Whats best in your opinion?
Click to expand...

I used scapa foam instead of sealant, as recommended by a friend. No mess. No leaks in 8 years. Check them at www.affixit.co.uk sales@affixit.co.uk
 
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