Leaking lexan skylights - butyl solution?

[NonTrawler]

Hi all,
This is my first post to the forum, so hopefully I'm doing it all right.


I'm hoping some you further along on the learning curve might advise me on a dripping matter. The matter at hand are leaking lexan skylights that are recessed in the cabin top of our (sail)boat. Originally fasteners were used every ~5-6 inches but the previous owner didn't like holes in the deck (which I appreciate) and went with an adhesive only approach to set and seal the skylight to the fiberglass and gelcoat. He used Sikaflex 295 for the job. According to him it lasts a year or so, but I'm thinking it's a lot less that that because it was redone a 6-7 months ago and it's leaking now. I can only assume it's a bonding issue or an elongation problem as there is some (appropriate, not excessive or spongy) flex in the cabin roof and in the lexan. The windows (ports?) are installed the same, but they don't seem to leak, probably because water isn't sitting on them like the skylight.


Sikaflex 295 seems to be spec'ed for organic glass (not sure what that is) which I pretty sure doesn't includes lexan . The rated elongation is 500%, which I think would be enough in this case. The Sika 295 is rated for UV, which is good because the joint is fully exposed to the outside.
I read a lot about butyl tape here and elsewhere, as well as Bed-It, the butyl-like option, and am pretty convinced that it would seal but it's not an adhesive so the skylight may not stay in place.


Anyway, this is where you all come in...what would you suggest I use once I remove the 295?

Is there butyl-tape that's adhesive enough?
Is there a butyl-based adhesive?
I'm guessing that 3M 5200 would work but I think it's more permanent than I would want. 4200 is not supposed to be used with lexan.
Is this an appropriate place for dreaded silicon?

I would like it to skin over so that people sitting on the cabin top would not be attacked by the sealant. It should be UV resistant as well.Oh, and not leak.

I've attempted to add three photos: 1 of the deck without the skylight, 2 with the skylight as currently installed.

Thanks in advance.
Mark

 

[Comodave]

Welcome aboard. I don’t think that butyl is the way to go and I love butyl. I use Sika 295UV for bedding lexan. I think the problem is the lack of a mechanical fastners. I would go back to the screws like the original design, the builder probably knew what they were doing... I would screw it down but not completely tight. Snug the screws enough to hold the lexan in place but not so tight as to drive all the 295UV out of the joint. Also bevel the screw holes so there is an O ring of the 295UV around the screws to seal them. If you want to use butyl you will absolutely have to use the screws to hold the lexan in place.

 

[STB]

Neither 4000 nor 5200 would be my 1st choice. They are expensive and comparatively inflexible. 4200 isn't really good in the sun.

I'm not at all a fan of home depot or lowe's solutions for marine applications. I'd never do it to save money. But, I have always had really good luck with Dynaflex 230 for sealing windows and seams at home. So, I started using it for some things on the boat. It hasn't let me down, yet. It is paintable and very flexible.
I just use the 230, not the Ultra, which I find too thick.

I set the replacement lenses in my Bomar 100 cast aluminum hatches with it, and they have no screws and are doing great, but they are only about a foot square. I did the same for a hatch I built for a prior boat. It was much larger and had no screws -- but had a huge recess to sit in.

Only time I every had trouble was one, single, crazy hot summer day working a large area, when it started to skin before I was ready.

Again, I recommend caution using a non marine product in a marine application, just because some guy on the Internet says it is okay...even if that guy is me.

 

[Comodave]

5200 will not stick to lexan or plexiglass for very long. Ask me how I know...

 

[TJM]

Have you removed the old lexan yet ? How is the bond between the lexan and the fiberglass ? That might lead you to the next step. If the sika is securely adhered to both materials then you may have an application issue. Maybe a prep issue with waxes ? Leak path ?

 

[Prospective]

I'd use Dow 795 or GE SSG4000 Ultraglaze. Both designed for structural glazing. The GE product is what bommar uses to glaze in their lexan hatches without any mechanical fasteners. Of course, prep is the key. Old surface has to be perfectly cleaned of all old product.

 

[fgarriso]

If you are sure you don't need to remove them again the 3M-5200 will do the job.

 

[skyhawk]

something that's common in the RV world is a product called eternabond.
To me it's a lot like butyl tape but permanently sticky. I have no idea if it's butyl of something else. It's available in different types, one of which is double stick...sticky on both sides.
I have no idea if it would work well in marine applications or if compatible with lexan, but maybe worth a look

 

[Comodave]

I have used Eternabond on my RV and the boat but I wouldn’t think it would work well in this application.

 

[NonTrawler]

Thank you all for thinking about this challenge.



First, an answer to the clarifying question:

TMJ: Have you removed the old lexan yet ? How is the bond between the lexan and the fiberglass ?

me: I can press up on the skylights in places and see the failing bond between the Sika 295 and the lexan. Unfortunatly I can't speak to quality of prep, but knowing the former owner, I'm assuming it was done correctly. Dangerous assumption perhaps, but I'm already in deep.



​

Thank you for the new suggestions to study up on. Eternabond, GE SSG4000, DynaFlex 230 (!), and a second dive on the Dow 795. I'd rejected the silicone-based products because of the challenges of surface re-prep when it comes time to replace the windows, but it may be worth it.


Interesting to note that DynaFlex 230 meets ASTM C920 class 25, same as the structural silicons Dow 795 and GE SSG4000, but it does not seem to be rated for marine use.



I think I'm going to go with one of the structural silicons and see how it does. I come across a lot of happy marine uses, similar to my application. I'm leery of using Sika 295 since that is what is not working now. I guess I'll look for other uses for butyl.



I sure appreciate the helpful suggestions.

 

[C lectric]

++++++++Sikaflex 295 seems to be spec'ed for organic glass (not sure what that is) which I pretty sure doesn't includes lexan . The rated elongation is 500%, which I think would be enough in this case. The Sika 295 is rated for UV, which is good because the joint is fully exposed to the outside.+++++++++

Organic Glass. Google it and you will find all it really means is clear plastics. I Googled it also. After 40 yrs in the plastics industry one way or another I had NEVER heard that term untill about a year or so ago.

Lexan [polycarbonate] is one of many organic glasses along with Plexiglass [acrylic] and others.

Preparation is paramount to any adhesive working in the application.

All plastics are ever so lightly waxy or oily. They are made of oils and when you heat them the oil shows.

They need a good wipe down to remove any trace of surface oil. First ALcohol would work. Then a light sanding to abrade the glueing surface, than another alcohol wipe down.
Chemical compatibilty chart. Some alcohols are better than others of course.
https://www.industrialspec.com/images/editor/chemical-compatibility-chart-from-ism.pdf

I see you have decided upon another product which is fine but prep. is still king.

 

[Prospective]

... I'd rejected the silicone-based products because of the challenges of surface re-prep when it comes time to replace the windows, but it may be worth it....

Normally I would agree with you. This is the lone exception in my book.

 

[STB]

If anything goes wrong using silicon, it'll be a lot of heartache to get a 2nd chance. Never mind the usual collateral if you get a tiny fingerprint somewhere that remains unnoticeable until paining or varnishing years later, etc.

There are great, very applicable products without this risk. Why take it?

 

[Xsbank]

Agreed. No silicone, no 5200. Both are capable of more damage than the perceived benefit of the repair.

The boat flexes pretty much continuously so you need a fastener system and a sealer. Try using the same stuff that manufacturers are currently using for gluing large windows in the side of swoopy running-shoe yachts?