Sealing Glass on Bronze Portholes

I am replacing the laminated safety glass in my portholes and replacing the old, leaky rubber seals.

Where the glass is held in the frame (sandwiched between two flat bronze flanges) I have several options for what to do...

  1. Cut silicone gaskets for each side of the glass
  2. Seal both interior and exterior surfaces with silicone sealant
  3. Cut silicone gasket for interior of the glass and use silicone sealant for the exterior seal
  4. Cut silicone gasket for interior of the glass and use bed-it butyl tape for the exterior seal
  5. I'm thinking about it wrong and I should ________________________________ to seal them
  6. Scuttle the boat and go drink in a bar
 

Zonker

Super Anarchist
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Canada
2. Some sort of sealant (doesn't have to be silicon) is way easier to compress and make a good seal. Just use lots of masking tape on the frame and the glass

 

Phtt

New member
Anything BUT silicone should work well. You only "need" to use silicone based caulk with some plastics (such as acrylic portlights). Silicone will work well, once, but then the  silicone contaminated surface will result in poor bonding in future, even to silicone.  So a polyurethane, polysulfide or polyether caulking OR butyl tape bedding would be better. The choice depends on the geometry of the joint.

I think of joints as being either "caulked" or "bedded", but this only my fantasy and I might be off base.

I consider caulking  (eg Sika 291) to be an elastic adhesive that must bond to two sides only, must have some thickness to accommodate the movement of the two sides, must have sufficient elasticity to allow strain (differential movement), can withstand pressure (strength) and needs a particular geometry (see sketch) to work properly.  The closer we can make the caulked joint look like the idealized example the more likely it will be successful. An added benefit is that the joint can just be cut with a knife for easy fitting removal. So, assuming the idealized joint is accessible, you can go ahead and use "devil's caulk" aka 3M 5200 because you can remove the fitting by just slitting the joint rather than trying to unbond the adhesive joint. You rarely need something so strongly adhesive though.  

Bedding (eg. Bedit butyl from Mainsail) on the other hand is a sticky gorilla snot smeared between two plates intended to fill the space so non-pressurized water cannot pass through. Caulking is often used as bedding, but can fail if there is insufficient thickness to accommodate strain between the two materials, whereas the bedding will just happily smush around to accommodate the differential movement. Bedding will not withstand much water pressure so would be inappropriate below the water line.  Bedding is easily removed if you have to remove the fitting or hatch to toe rail. 

A gasket is a different beast and requires that the surfaces on both sides not deform when bolted together else the space between the bolts will bear unevenly on the gasket with potential leakage. So, a rigid bronze casting pressing on a gasket resting on a cabin side should be good. A thin ring, on the other hand,  bolted against a gasket might leak and you would be better to use bedding. Of course, a thick soft gasket can work to withstand low pressures as it can deform with the deforming thin ring.  A good gasket example is an engine head gasket sandwiched between two rigid plates (the head and block). You likely have two rigid surfaces so a soft thick silicone or rubber gasket might also work well and be less messy than caulk or bedding. 

I will let the more mechanical types correct anything here that is goofy. 

caulked joint_000001.jpg

 

SloopJonB

Super Anarchist
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Great Wet North
Use Dow 795. It was originally intended to hold the huge panes of glass on curtain wall high rises so it should be able to hold a few square inches of glass in a porthole.

 

Diarmuid

Super Anarchist
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Laramie, WY, USA
Use Dow 795. It was originally intended to hold the huge panes of glass on curtain wall high rises so it should be able to hold a few square inches of glass in a porthole.
Just bought three tubes of grey Dow 795 to re-bed our (fixed) port glass & aluminum frames. (Three tubes because somebody on eBay was trying to shift excess  stock; it's a good place to find sealants!) You can buy it in about seven colors, it doesn't need priming or UV masking, it won't ooze in hot sun, it's the perfect balance between sticky adhesive & resilient gasket, and from building our foredeck hatch I'm pleased with its ease of application. (Gotta clean up as you go, tho!)

 

IStream

Super Anarchist
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A couple layers, too. The stuff is so stringy that the first layer can leave threads on the second.

 
I notice that y'all are putting the seals on the INTERIOR surface of the glass. Is this common? Is there some reason for it? Why not put the seal on the outer surface to stop the water from getting into the screws? Or better yet, both?

 
Pfffft has some good input on the hell silicon can cause, have never used butyle but seems like what the masses like.  Would probably not use any of the poly's.  Even with a perfect instal the life on it is sun is not super great. We used thinned dolfinite on fixed safety glass windows about 8 years in 3 in tropics and time to think about maybe re-bedding. Cheap but not for all DIYS.

 

Diarmuid

Super Anarchist
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1,593
Laramie, WY, USA
I notice that y'all are putting the seals on the INTERIOR surface of the glass. Is this common? Is there some reason for it? Why not put the seal on the outer surface to stop the water from getting into the screws? Or better yet, both?
Opening ports must have the sealant on the inside frame. Or they don't open. ;)

Otherwise, it sorta depends on the port construction.  Some deadlights pin the glass or plastic to the hull's outside, some to the inside, some 'float' the pane captive between two frames. You put the sealant where it does the most good.

 
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