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Old 21-08-2015, 13:34   #31
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Re: Recommended Sealant/Adhesive for Installing Glass Windows

Originally Posted by sailjumanji View Post
Side Windows is what I am working on now. Panels are glass. No screws on them. I need something to hold the glass vertically in place while sealant dries. There is almost a two inch wide recess behind each window which is filled 1/4 inch thick with sealant. I need to use tape or something , fill the window recess, then out the window on and clean up.

So is the butyl sticky enough to hold the window until sealant dries.
So the answer to the last question is. depends. My feeling is yes I think it would hold. But depends on heat and weight and lots of factors. But in general yes I think it would. It would help if you maybe got a suction cup or something and stick it in the middle and tied it off to just keep the weight of the window from causing it to slide down. Once you put the butyl tape around the edges and put the glass in you will be able to tell what else you might need. Even if you have to put a couple of spacers along the bottom you would still be OK. After it all dries just go back pull the spacers and fill with 795. The butyl will certainly hold the glass to the boat but it might let it slide down during the cure time for the 795....might....but easily solved I think if it did.

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Old 22-08-2015, 10:59   #32
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Re: Recommended Sealant/Adhesive for Installing Glass Windows


To answer your original question,(and as much as I and most "boat" people hate silicone) Dow 795 is the correct product to use on glass( GE SG4000 is recommended as well), and is available in black. Should be easy to find, my local glazier had black 795 on the shelf and I live in a very small town. It's cure time is 7-14 days via atmospheric moisture, so you'll need some way to brace the glass. Make sure it's within its best before date as some people have had issues with expired product. I would check with a glazing shop for black tape. I'm in construction and have seen it used by glaziers on our sites. I agree with not doing a spot fix so doing a temporary seal until you get home when you have the time and access to tools and products might be a good idea. Good luck with the repairs.

I'll repost this excerpt from Maine Sail once again, because it's important to keep water on the outside of the boat!
The experts he's quoting have many decades of experience....

Dow 795 is what many builders and manufacturers use to bed. It is a structural glazing silicone and is widely available.

We have had one manufacturer join this thread, and one who BTW makes excellent ports and hatches, of very high quality, and the advice given is excellent.

Below are some quotes from Tony D'andrea who is the Acrylic & Polycarb guru in the marine industry. He is the Nigel Calder of portlights and hatches and owns the largest hatch warranty & repair facility in the world called Select Plastics. He also worked for Cyro the makers of Acrylite for a number of years and really knows his stuff.

As a racer I have seen just about every thing from a spin pole to a winch handle dropped on cast acrylic hatches, most hatches with the exception of a couple of Bomar's (I think the 100 series with the support bars) are cast acrylic. Not once in my 30 years of racing sail boats have I seen an acrylic hatch crack or shatter. That's a LOT of shute launches and a lot of people on deck standing on hatches.

My lenses in my Goiot hatches were finally replaced last spring at the 30 year mark. One of them is 26" X 26" and had well over 30k nautical miles put on it by a good friend who owned it before me. She went from South America to Europe to Labrador to Alaska and back.

In 30 years this 26" X 26" cast acrylic never once leaked nor did it crack or show any signs of it. It was walked on by 300 pond individuals and had been whacked by numerous items over the years. While it was not all to clear at 30 years I could still actually see out of it..

Tony D'andrea on Acrylic vs. Polycarbonate:

"Consider the Following:

  • All major hatch, portlight and window manufacturers use Acrylic in offshore / bluewater marine products.
  • Acrylic is more scratch resistant than standard (9034) polycarbonate.
  • Acrylic is significantly more durable when exposed to Ultra Violet radiation (sunlight).
  • Acrylic is less expensive than Polycarbonate.

Don't misunderstand my preference for acrylic. I buy, use and sell a significant amount of both products and each has its application. In my humble opinion Acrylic is more durable, versatile and cost effective in the hands of a skilled craftsman than polycarbonate.

Additional considerations may include polycarbonate with UV and scratch resistant coatings. While these products are heavily promoted by several manufacturers and carry 5, 10 even 15 year warranties the following information has been reported in "real life"applications:
  • Polycarbonate is impact resistant. When its new it is almost impossible to break.
  • Small quantities (less than a 4 by 8) in gauges over 1/8th inch are difficult to to find in the uv/scratch resistant grades.
  • Colors are limited. Only two standards (gray and bronze). Try and find anything thicker than 1/4 in UV/ scratch resistant!
  • Polycarbonate foreshortens when subject to static or dynamic loads. What this means is if you replace your hatch lens with polycarb, seal it and then step on it the ductile material will deflect (bow) in the center. One of two things may happen. 1st you will surely break the watertight seal, 2nd you may end up with a leg in your galley.
  • As for the warranty: The original owner is warranted against failure subject to the material being submitted to the distributor for evaluation with the original invoice subject to actual replacement cost at the time of purchase. I guess this means they sell you a new square of material and apply the old payment to the new cost. How about the labor to fabricate the part, install it and sealant? Why take the chance?
Polycarbonate is a great material, The US Air force uses it for fighter canopies! I sell Polycarb to the USCG and US Navy. Remember they don't mind using it because we are paying to replace it every three years.
Both Acrylic and Polycarbonate have specific uses and installation requirements.

Cast Acrylic (of a specific thickness) is in accordance with CE and ABYC guidelines, and installed on virtually all of the big blue water sail boats produced on both sides of the pond. Polycarbonate is commonly used as a replacement due to its ease of fabrication and incredible initial strength. The USCG and USN require Polycarbonate on their vessels but they also have a PM cycle of 36 to 42 months for change out. My Tax dollars at work...

Due to its ductility Polycarbonate it is more challenging to install. I have seen Sika Flex 295UV with primer and Dow 795 both mentioned. I use and recommend both. Dont go over 4 ft continuous length with a fixed portlight. Remember the coefficiant of thermal expansion for Acrylic and Polycarbonate is in the neiborhood of .000039 per inch per degree F. That means an 8ft plastic port will expand and contract up to 1/2 of an inch from the coldest day in Feb to the hottest day in summer. WOW!! Compartmentalize the job. It will be easier to install and less prone to leaks.

Never ever bolt a plastic portlight in place. Screws are fine to hold a lens till the adhesive cures. Take them out asap and fill the holes with the afformentioned products. Both of these products are rated at 700 + percent elongation before tear.
Strong flexible and UV resistant.

Been to a boat show lately? Seen any screws? Glass is good so long as your boat does not twist or torque. Show me a fiberglass boat that does not twist and I will show you a cocktail barge tied to the dock.

Tony on Sealants:

"I hear the question as to which sealant to use when bonding Acrylic, or Polycarbonate to aluminum, stainless or FRP over and over and over.....
Well here goes... The only three adhesives I would consider using are Sika Flex 295 UV with the primer, GE SG-4000, and Dow 795. Using the correct adhesive is only 1/2 the battle. Do not apply the sealants below 50 degrees F. The temperature must maintain at least 50F during the entire 21 day cure cycle. Cut this corner and your finished before you start. Preparation of the bond area is also very/ very important. DO NOT TOUCH THE BOND AREA WITH YOUR BARE HANDS! Contamination from the dirt and skin oils will make a solid cure impossible. You may clean the FRP and metal with acetone to prep the area but if you touch the Lexan or Plexi with harsh solvents you will ruin the portlight. A 50/50 mix of isopropal alcohol and distilled water will work well to clean the plastic if needed. Remember that clean enough is not clean enough." __________________

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