Replacing portlights with screwless, VHB tape mounted windows

[miketcook]

I am replacing the four portlights, and had new plexiglass windows cut from the originals. Am using 3M VHB tape and Dow 795. But found the new windows, cut exactly to old windows were too big to seat flat on all four sides against the 3M tape - so no seal. Thought trimming them 1/8" all around would do - didn't. So am going to measure dimensions of FLAT window frame, where tape will make contact.
Guess it's obvious in hindsight that I should have measured first, although someone said to just bring in old windows. Anyone else have this experience? Or everyone knew to just measure each frame exactly for the VHB mounting ??
Am hoping that using the Dow to caulk between the window edge and up to the slanted edge of the window frame will seal well?
PS have a lot of holes (40) - all the way through to cabin, that I'll have to clean and drill bevels into, to seal with Duraglas before taping and mounting. Lots of interior work to follow up with.

 

[kloudie1]

Some folks have used temporary screws (as few as possible) to hold the frameless glass in place until the adhesives fully cure. They then remove the screws and fill the holes with more 795.. The VHB takes a few days to fully cure and the 795 can take a couple of weeks.. Some folks have used blocks and braces to hold the plexi down while the cure happens. The VHB is "closed cell" so it is watertight after curing. It is crucial that the surfaces of the cabin (especially where any silicone sealant may have been used) and the plexi be meticulously cleaned and prepared and then primed (3M, #94 primer) to get a really good bond. When I did my H-34, the front of the 1/4" plexi was about i/4" away from the cabin side. It bonded strongly enough to not require me to use temporary retaining screws. The plexi was cut so as to cover the old frame screw holes with VHB
.

 

[d-marko]

What year and type of windows do you currently have? I replaced the windows on my ‘88 mk2. The original windows had a bevel. When I cut my own plexi windows I trimmed them to eliminate the bevel so they would sit flush in the window cavity. And filled the area the bevel previously occupied with 795. Perhaps this is the cause of your issue?

I didn’t bother filling the screw holes. Just scraped out any loose fiberglass and taped over them.

also I found that the windows did need some pressure applied for the VHB to seal well. I cut some wood strips to size and wedged them in between the windows and the stanchions to apply pressure on the plexi against the VHB

 

[Ralph Johnstone]

Some folks have used temporary screws (as few as possible) to hold the frameless glass in place until the adhesives fully cure.

Have to disagree with you just a tiny bit here. Temporary screws are just NOT a big deal when installing frameless glass. They are near invisible. My new 1999 H-310 came from the factory with lots of screw holes in the glass and you have to press your nose up against the glass to see them as they are filled with black 795.

See how many holes YOU can find and win a kewpie doll.

 

[BigEasy]

If you are using adhesive only, I understand the need for screws or cleats to retain the glass while the adhesive cures. I recently replaced a deck skylight on my boat that is approximately 5' x 2'. As Kloudie mentioned, VHB tape was sufficient to hold the glass in place without using screws. Very delicate operation because once the tape contacts the glass and mounting surface, you are not going to remove it. Dow 795 was used to seal the edges to prevent water intrusion and for cosmetics. My concerns about placing screw holes close to the edge of the glass is that you are compromising the integrity of the glass. It is a fact that the lens expands and contracts substantially with heat and cold exposure. Those screw holes are likely to cause fractures over time. Beneteau's field guide to replace the lens, advised use of wooden cleats with screw holes into the fiberglass deck; they advised to not drill the cast acrylic. Note that Beneteau did not use VHB tape; they only used adhesive.