# Which sealant?



## copacabana (Oct 1, 2007)

I would like to remove and reseal my portlights. I know there are older threads on the topic, but new products come on the market and ideas change. Besides, no one has mentioned gaskets in the threads.

The portlights on my boat have no frames. They are thick plexiglass lenses bolted to the cabin sides. The overlap is fairly generous (big bolts). I think they are currently sealed with silicone (and starting to leak). I was thinking of using rubber spacers (gaskets) around the bolts to keep the lens a few millimetres off the cabin sides in order to leave more sealant. I think the sealant gets squeezed out when the bolts are tightened and then shears and leaks when the lens and cabin sides contract and expand at different rates. 

The sealants I can get locally are Sikaflex 221, 291 or 295UV. Which would be the better choice? I'd like to have really leak-free ports that last.

Also, are the rubber spacers (gaskets) to hold the lens a little off the cabinsides a good idea? 

Thanks in advance,

Mark


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## Maine Sail (Jan 6, 2003)

copacabana said:


> I would like to remove and reseal my portlights. I know there are older threads on the topic, but new products come on the market and ideas change. Besides, no one has mentioned gaskets in the threads.
> 
> The portlights on my boat have no frames. They are thick plexiglass lenses bolted to the cabin sides. The overlap is fairly generous (big bolts). I think they are currently sealed with silicone (and starting to leak). I was thinking of using rubber spacers (gaskets) around the bolts to keep the lens a few millimetres off the cabin sides in order to leave more sealant. I think the sealant gets squeezed out when the bolts are tightened and then shears and leaks when the lens and cabin sides contract and expand at different rates.
> 
> ...


Mark,

Dow Corning 795 & Sikaflex 295UV (WITH the special primer) are probably the two most widely used products for this. GE also makes SG-4000 which would also be appropriate.

The real trick and key to this is to get the surface 100% clean of silicone contamination. This is easier said than done. It will almost always require wet sanding. You could also use butyl tape as your gap product and 795 or 295UV around the outer edges if the overlap is big enough. You DO need enough sealant to allow for expansion & contraction. Generally through bolting polycarbonate or acrylic is contraindicated but if the holes are big enough to allow for movement it can sometimes work without cracking the ports/dead lights.


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## copacabana (Oct 1, 2007)

Thanks for info Maine Sail. I'm a big fan of your how-to articles. I thought of using butyl, but I can't find it here in Brazil. I'm left with SIKA products, which are readily available. I guess the Sikaflex 295UV is the best option then. Could you please elaborate on what you mean by "WITH the special primer"? Do you mean that there is a special primer to apply to the surfaces before applying the sealant?

Concerning the holes, they are oversized. The windows have been on this way for 15 years and haven't cracked around the bolts so far. I am assumng the original owner got the hole sizes right. Do you see any advantage in using the spacers to assure a thicker gasket of sealant between the lens and cabin sides? I can't help but think the sealant will be all squeezed out when I tighten the bolts. 

Would acetone remove any residual silicone? 

Thanks again.


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## Maine Sail (Jan 6, 2003)

copacabana said:


> . Could you please elaborate on what you mean by "WITH the special primer"? Do you mean that there is a special primer to apply to the surfaces before applying the sealant?


When bedding polycarb or acrylic Sikaflex has a special primer to help the PU bond to the plastics. It really MUST be used when doing this job or you will likely leak again.



copacabana said:


> Concerning the holes, they are oversized. The windows have been on this way for 15 years and haven't cracked around the bolts so far. I am assumng the original owner got the hole sizes right.


good



copacabana said:


> Do you see any advantage in using the spacers to assure a thicker gasket of sealant between the lens and cabin sides? I can't help but think the sealant will be all squeezed out when I tighten the bolts.


Yes but I would not do them around the bolts as this is a critical point of water ingress and the bolt holes through the Fg should be chamfered so they can fill with sealant. You can shim with double sided foam tape, plastic floor tile spacers that can later be pulled out, rigid foam weather stripping or many other products but just make sure you still have enough raw surface area for a good strong seal and that your spacers do not interfere. I have used floor tile spacers a number of times. You only need about 1/16th of an inch of penetration into the sealant interface to support it until cured.



copacabana said:


> Would acetone remove any residual silicone?
> 
> Thanks again.


No. I know of no product that will remove silicone contamination from gelcoat without also destroying the gelcoat. Even new silicone does not stick to old silicone contamination. This is why so many folks despise silicone for use in the marine environment. Sikaflex even specifically contraindicates against bedding over old silicone in their technical data sheets for PU sealants.


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## killarney_sailor (May 4, 2006)

The Sika primer for 295UV is called 209N which you paint on the surface. It is ridiculously expensive ($60 for 500 ml as I remember) and has a very short shelf life after you open the can but it works really well. I used the sealant without the primer and it did not work - had leaks. Redid it with the 209N and it was great. Sika also suggests a particular cleaner; did not use this but did some sanding and cleaning with some strong cleaners I had and it seemed to work fine. BTW, one more thought about the 209N, check the expiry date on the can before you buy - the first couple of places that had the primer had cans that were 2 to 4 years out-of-date.


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## casioqv (Jun 15, 2009)

copacabana said:


> I thought of using butyl, but I can't find it here in Brazil.


I'd probably use butyl tape, because it will still seal if there's some movement between the surfaces.

I'm certain it's possible to find butyl tape in any country which has automobiles- because it's used for sealing in replacement windshields. This stuff is black, which works but is messier than the grey stuff.

If not- you can purchase it online:
Heavy - Duty Butyl Tape, Gray, Rv, Brand Not Specified at Sportsman's Guide


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## copacabana (Oct 1, 2007)

I was led to understand the automotive butyl was an adhesive, not a sealant. I have read that the automotive butyl for gluing in windshields was not recommended for sealing portlights. Maine Sail, could I use the automotive stuff? It would certianly be cheaper and easier ...


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## Maine Sail (Jan 6, 2003)

copacabana said:


> I was led to understand the automotive butyl was an adhesive, not a sealant. I have read that the automotive butyl for gluing in windshields was not recommended for sealing portlights. Maine Sail, could I use the automotive stuff? It would certianly be cheaper and easier ...


If they are through bolted butyl can work but would not be my first choice for a dead light, and I sell the stuff so just being honest. It has no contraindications with either polycarb or acrylic but would likely ooze out over time with the gasket thickness needed unless you put in permanent, left in place shims. If using it around the interior perimeter and sealing the outer with 295UV it would work well and you would avoid ooze out. You would want 1/6" or 1/8" thick by 1/2" or 3/4" wide depending upon how much real estate you have to cover width wise. You would also want gray butyl not round black. I would love to sell you some but I can't ship to Brazil. I would still use 295UV or Dow 795 for deadlights..


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## copacabana (Oct 1, 2007)

I think I may have found the grey butyl here made by Sika (Sika Lastomer 955). Maine Sail, my portlights are through-bolted (you mention above). They are very securely fixed in place by many big bolts. Could I use this grey butyl? The Sika primer and 295UV route is looking very expensive right now. A 300ml tube of 295UV is about 35 US dollars and I'm sure the primer isn't going to be cheap here either. I would imagine one tube would do about 2 portlights and there are quite a few to do ... I had never heard about butyl leaking out between the lens and the cabin sides. Is it that soft or does it get soft in the sun? Sorry to pester you with so many questions, but I can't seem to get straight info here (everyone here tells me to use silicone bought at the hardware store!).

Thanks again.


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## copacabana (Oct 1, 2007)

Is this the same as the grey butly?

Tyvek® Construction - Tyvek® Butyl Tape


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## QuickMick (Oct 15, 2009)

copa you speak (er write) in english very well. Any way if you go the butyl route you may be able to find it at a place that sells/services/rents campers. a lot of those have butyl tape in their windows and i found mine at one of those stores... just didnt wanna wait for the on line shipment. not sure about that link. mine was greyish puttyish with a waxpaper backing to prevent self stickage, unless that 'silverish' look is from the backing, that almost appears to be for metal applications? good luck,Q

ps. mine looked more like this

Amazon.com: 3/4" X 30' Butyl Tape: Home Improvement


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## ffiill (Jul 15, 2010)

Remember when bedding down an item such as this do not fully tighten your screws/bolts .Allow the sealant to cure then carry out a final tightening-this effectively creates a rubber gasket which you finish tightening creating a perfect seal whether or not the sealant has bonded to the acrylic/glassfibre..
I have used low modulus silicolne sealant to great effect so long as surfaces are absolutely dry.
Also the brands you refer to do have trade unbranded equivalents-just need to research products online.


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## copacabana (Oct 1, 2007)

Thanks everyone for the input. I feel I have a few options now. Ideally I'd like to do one portlight a weekend, so it doesn't feel like a big job and interfere in my fun time with the family. This sort of rules out the Sika primer route (I think) as it has a short shelf life after opening the can. Also, I think the Sikaflex 295 probably dries up as well if left open. If I can get the grey butyl tape, I'll try that on the first portlight to see how it goes. Maine Sail, I'll take your advice and use a permanent shim to hold the butyl in place. You seem to recommend two-sided foam tape, which I believe is easy enough to find here. If anyone else has any advice, please fire away... I still have 2 weeks before I do the first portlight. I'll take photos and report back with the results, one way or another!


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## SJ34 (Jul 30, 2008)

I've been using Sika products for over 20 years and have always been happy with the results when they were used correctly. I re-bedded my ports 3 years ago with 291 and still no leaks and still flexible. Works kinda like butyl that way. Also, the local yard uses the 291 on frame-less ports when the vessel sees immediate use because it's a moisture cure product and creates a seal almost immediately.


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## Annapolisboater (Jan 1, 2007)

*The best choice is Silproof*

We have been repairing and refabrication/installing ports & hatches for over 30 years. Silproof has the chemical make-up to afford the physical flexibility to match the expansion & contraction of the ports.


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## ffiill (Jul 15, 2010)

Appears to be a branded low modulus silicone sealant-exactly what I have come to use through personal experimentation over the years.
When it cures its just like rubber and far less expensive than the products mentioned here.
I buy LM silicone in UK by the box (20 or so at a time) and it works out at well under $2 a tube.
It sticks to glass and glassfibre like glue! -be warned .I also use it for some interior timber gluing;vinyl flooring;great on boat vinyl headliner. I also use it instead of proprietry brand known over here as "no more nails"


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## copacabana (Oct 1, 2007)

Annapolisboater, Is silproof a brand name? Do you happen to knpow if 3M or Sika have a similar product? I'm not in the US and the products here are different. Thanks.


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## Faster (Sep 13, 2005)

copacabana said:


> Annapolisboater, Is silproof a brand name? Do you happen to knpow if 3M or Sika have a similar product? I'm not in the US and the products here are different. Thanks.


Mark, it's a GE product tradename "Silpruf"...

GE SilPruf SCS2000 Silicone Sealant by Piedmont Plastics, Inc.

FWIW I've had luck on several occasions on several boats with DC 795 as recommended by Mainesail. Others have indicated that DC 739(793?) has better adhesion but I've not used it.


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## copacabana (Oct 1, 2007)

Thanks Ron. As you know, my selection of sealants here is somewhat 'limited'. I'd like to try the Dow Corning 795 or is it 739?), but I can't find it. I think I'll end up doing one port with butyl and a gasket of 2-sided foam tape (as Maine Sail suggested). If it works, great. Then I'll do the others one at a time on weekends. If butyl can give a permanent, waterproof seal for windshields, even with 120km winds and rain, it's got to be better than the 'marine' silicons that have been used on the boat before. 

I'll try to post some pics over the next few weeks to give a follow up report (either with a big smile on my face or hanging my head in shame...)


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