Anyone Else Using Plexiglass Deadlights With VHB Tape?

[s/v Jedi]

Originally Posted by 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.

Yes, there must be a thread somewhere explaining these methods incl. routing a rebate for sealant bead thickness.

Also, you can use closed cell foam/rubber on the flange opening so that this closes the gap seen from the inside so that no dirt/debris gets into there. The VHB tape goes around that and 795 sealant around that after window is in place.

[marcjsmith]

i have done this with my moody 376

remove the windows and the frame tape off the opening straighten up the cut lines then vhb tape and the dow 795

[s/v Jedi]

Originally Posted by marcjsmith

i have done this with my moody 376

remove the windows and the frame tape off the opening straighten up the cut lines then vhb tape and the dow 795

This looks great but there is a potential long term problem: the windows are mounted proud of the cabin sides instead of flush. For this type of installation I recommend to paint the cut edges of the polycarbonate or acrylic with black paint like Easypoxy. You want to prevent UV entering the window through this cut edge, because the window front and back will reflects the UV back into the window again and again, like a fiber optic transporting light. This accelerates UV degeneration greatly.

I have this same problem and am planning to first glue some G10 strips around the openings so that I can install the windows flush with those strips, protecting the cut edges.

[Jammer]

I used VHB and Dow 795 when rebedding a plexiglass light on my 1983 Hunter 26, and was pleased with the results.

[marcjsmith]

Originally Posted by s/v Jedi

This looks great but there is a potential long term problem: the windows are mounted proud o

Yes windows are proud... the edges are not flamed they are frosted and when I ran my bead of 795 around the edge Ive got a bevel of the 795 that covers the frosted edge.

[s/v Jedi]

Originally Posted by marcjsmith

Yes windows are proud... the edges are not flamed they are frosted and when I ran my bead of 795 around the edge Ive got a bevel of the 795 that covers the frosted edge.

That’s what I have, but over time, the silicone will pull back and expose the edges. Frosted is even worse because it doesn’t reflect any incoming light. We now have vertical 4”-6” wide lines through the polycarbonate originating from spots where silicone pulled away. Trying to repair didn’t work because silicone doesn’t adhere to cured silicone.

After 1-2 years you can already see it when you hold a new (cutoff) piece of material next to the window. A friend who painted the edges reported spotless windows after many years.

[marcjsmith]

Originally Posted by s/v Jedi

After 1-2 years you can already see it when you hold a new (cutoff) piece of material next to the window. A friend who painted the edges reported spotless windows after many years.

thanks for the info. I'll take a look at my edged and see if I have any issues with the silicone pulling back. Its been a bit over one year since Ive done mine...

[akuakai]

I used vhb tape and 795 on plexiglass on a recent catamaran build. In addition to comments above, I found making sure the 795 is somewhat warm, as in sitting in the sun for awhile, prior to application. Makes it flow behind the plexiglass a little easier. And avoiding application in direct sunlight where it will dry before you get a chance to tool it. It looks the best if you can make the lap around the window in one pass with the caulk gun and tool the joint (or finger it) in one pass. And pull the tape before it skins over.

The clock is ticking.

[Notfast]

Originally Posted by akuakai

I used vhb tape and 795 on plexiglass on a recent catamaran build. In addition to comments above, I found making sure the 795 is somewhat warm, as in sitting in the sun for awhile, prior to application. Makes it flow behind the plexiglass a little easier. And avoiding application in direct sunlight where it will dry before you get a chance to tool it. It looks the best if you can make the lap around the window in one pass with the caulk gun and tool the joint (or finger it) in one pass. And pull the tape before it skins over.

The clock is ticking.

Thanks, Broseph!🤝