Which sealant?

[copacabana]

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

[Maine Sail]

Originally Posted by copacabana

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

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.

[copacabana]

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.

[Maine Sail]

Originally Posted by copacabana

. 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.

Originally Posted by copacabana

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

Originally Posted by copacabana

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.

Originally Posted by copacabana

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.

[killarney_sailor]

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.

[casioqv]

Originally Posted by copacabana

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

[copacabana]

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 ...

[Maine Sail]

Originally Posted by copacabana

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..

[copacabana]

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.

[copacabana]

Is this the same as the grey butly?

Tyvek® Construction - Tyvek® Butyl Tape