Anyone Else Using Plexiglass Deadlights With VHB Tape?

[Notfast]

I just bought a bunch of tinted plexiglass for my deadlights and am planning on installing it using 3M VHB tape and nothing else. Supposedly this provides a superior seal compared to traditional framed windows. Any tips? Advice?

I'm using 1/4" thick plexi on deadlights consistent with those found on an Albert 30.

[s/v Jedi]

Originally Posted by Notfast

I just bought a bunch of tinted plexiglass for my deadlights and am planning on installing it using 3M VHB tape and nothing else. Supposedly this provides a superior seal compared to traditional framed windows. Any tips? Advice?

I'm using 1/4" thick plexi on deadlights consistent with those found on an Albert 30.

Afaik VHB tape is an adhesive, not a sealant. Use VHB tape to set it in place, then caulk around the perimeter for sealing. This is actually one of the few places a structural silicone like Dow Corning 795 shine.

[Artey]

I have not done it with VHB only, but I have done it with VHB and followed by Dow Sil 795 and this is an amazing combo. You might be okay with just the VHB but I'd be mostly worried about thermal expansion, not the seal of the tape. Once the thermal expansion creates a problem, you are back to leaking.

[Notfast]

Originally Posted by Artey

I have not done it with VHB only, but I have done it with VHB and followed by Dow Sil 795 and this is an amazing combo. You might be okay with just the VHB but I'd be mostly worried about thermal expansion, not the seal of the tape. Once the thermal expansion creates a problem, you are back to leaking.

Yeah actually I intend to use Dow 795 silicone around the edges like you did. I just meant I won't be using bolts or frames with the new windows. Me not write so gud 🤪 haha

How long have you had your windows in and leak free?

[GordMay]

Originally Posted by Notfast

I'm using 1/4" thick plexi

Yeah actually I intend to use Dow 795 silicone around the edges like you did.

IIRC most small to medium sized hatches come standard with 10mm [just over 3/8 inch] thick Acrylic lenses. Larger hatches might use 12mm [just under ½ inch].
Sika Flex 295UV with primer is also good.

[Hudson Force]

Originally Posted by s/v Jedi

Afaik VHB tape is an adhesive, not a sealant. Use VHB tape to set it in place, then caulk around the perimeter for sealing. This is actually one of the few places a structural silicone like Dow Corning 795 shine.

I followed this advice from Jedi and was very pleased with the result. Plexiglass is an acrylic and I favored polycarbonate instead. There are pros and cons with each and it's important to choose what's best for you regarding crazing, scratch resistance, flexibility, etc.

[Martkimwat]

From what I have been told and have gleaned from my own experience, it is important to try and stop the plexiglas from being pressed right up to the surface you are sticking it to, try to maintain a 3mm (1/8" inch adhesive and sealant thickness. This allows the two surfaces (plexiglass and backing surface) to expand and contract at different rates without breaking the bond. On bolted on windows, I have achieved this with 3mm spacers on the bolts between the plexiglass and the surface. Not sure how you would achieve it without fixings but I'm sure it's doable. Depending on how much overlap you have, you might be able to stick a 3mm x 3mm plastic batten on the inside of the plexiglass or on the GRP surface that coincides with the frame cutout so as to provide a neat trim surface on the inside. Strips of ABS plastic would do the job, available from hobby stores or model shops, usually in white or black, or you could just put four strategically placed bits on the surface first to make sure it hold s the plexiglass off the surface, especially if there is no curvature to worry about.

[bcboomer]

Originally Posted by Martkimwat

From what I have been told and have gleaned from my own experience, it is important to try and stop the plexiglas from being pressed right up to the surface you are sticking it to, try to maintain a 3mm (1/8" inch adhesive and sealant thickness.

This why it is important to use 2.3 mil 4991 VHB tape. You don't need spacers as the tape itself provides the necessary gap for the Dowsil 795.

Detailed instructions are here:
https://www.sailmagazine.com/diy/rep...xed-portlights

[Martkimwat]

Originally Posted by bcboomer

This why it is important to use 2.3 mil 4991 VHB tape. You don't need spacers as the tape itself provides the necessary gap for the Dowsil 795.

Detailed instructions are here:
https://www.sailmagazine.com/diy/rep...xed-portlights

Oops, I missed that it was a tape, I am not familiar with the product and thought it was adhesive, as someone else mentioned. Is it an adhesive both sides closed cell tape? We don't have anything like that here in UK. It would be very useful. Who makes it?

[bcboomer]

3M make it. It should be available.

FYI, they also have a new tape for slippery plastics that I used to attach flexible solar panels to a bimini frame. It works well.

[s/v Jedi]

For VHB tape, here is 1” wide but you can get it in many different widths:

https://www.amazon.com/3M-Double-Sid...s%2C280&sr=8-3

[Lake-Effect]

Originally Posted by Martkimwat

From what I have been told and have gleaned from my own experience, it is important to try and stop the plexiglas from being pressed right up to the surface you are sticking it to, try to maintain a 3mm (1/8" inch adhesive and sealant thickness. This allows the two surfaces (plexiglass and backing surface) to expand and contract at different rates without breaking the bond. On bolted on windows, I have achieved this with 3mm spacers on the bolts between the plexiglass and the surface.

We removed, cleaned and reinstalled the acrylic "glass" on our microcruiser. PO had used butyl there and it attracted all manner of dirt and grunge. The lights are bolted on. We used clear silicone as the new sealant, and as screws were snugged up (not fully tightened), we slipped toothpicks in from inside the cabin, going in approx 1/8" between the cabin and the acrylic, to maintain a toothpick's thickness (roughly 1 to 1.5 mm) of silicone, and scraping/wiping off the excess. Once the silicone had cured a couple of days, we removed the toothpicks and tightened the screws a bit. This has been clean-looking and leak-free for about 9 years now.

[BAD ORCA]

You can make the bond even stronger/ water tight by etching the acrylic wherever the 795 is going to be applied. I laser etched my hatch lense where the 795 was going to make contact, but i've also masked off my hatch with blue tape and sanded the area with #220 sandpaper where the 795 was going to make contact. It improves adhesion greatly.

[Bycrick]

Five opening deck lights with aluminum frames. I finally settled on just DC 795 without the VHB tape. I used 1/8” bamboo skewers to space the light away from the frame. The caulk outlasts the acrylic and I’ve never had a leak, even with a lot of sea time.

[PegasusIX]

Yes I used VHB tape for dead lights and large deck window. Worked great. The amount of differential thermal expansion between the acrylic and boat fibreglass needs to be considered. The 2.3mm VHB tape’s thickness allows the Acrylic to move relative to the cabin sides, to expand, and tinted black Acrylic will get hot in the sun. Deck window had failed previously as it was over 2m in one piece, new one was 3 pieces joined over a rail, with 10mm gap, sealed. VHB holds the acrylic in place and sealant around the outside keeps the seawater away from the sealant. If you install the VHB in one piece with a join at the bottom it is the second line of defence against water. You can cut small triangles out of inside of VHB to go around curved corners but still restrain continuous VHB for inner water seal. Yes 3M recommend sanding Acrylic mating surface with 100 grit to increase surface area to improve adhesion. They also recommend wiping ‘one way’ with 50:50 isopropyl alcohol and water mix to clean both surfaces. Apparently if you wipe two way it takes off any contaminates and then you wipe it back on! I added a rebate to the edge of my windows to increase sealant thickness from tape only 2.3 to 6mm or so. 5mm x 5mm closed cell foam in the bottom of rebate with sealant over. Larger sealant size allows more movement from differential expansion. If you want to get really scientific you can calculate likely expansion of acrylic and fibreglass over expected temperature range, taking into account full sunlight likely surface temp of materials. The longer the Acrylic the greater the expansion.