3M VHB - Need Primer for Acrylic?

[Cdn_bacon]

Curious if you need the 3M Primer 94 on either the acrylic or fiberglass (gelcoat or non-gelcoat) for bedding windows?

[Spot]

Which VHB?

https://www.3m.com/3M/en_US/vhb-tape...duct-selector/

[Chotu]

Yes. You should absolutely use primer 94 if you want to get the best results.

[paul2884]

Just been reading the spec on the tape, it looks like a great product, be very interested in how you get on with it.

[Cowpoos]

Yes, and I would still screw the acrylic based on my experience.

https://www.youtube.com/watch?v=grluO5gZQYg&t=1s

https://www.youtube.com/watch?v=G6EP7fWJp1I

[Chotu]

Originally Posted by Cowpoos

Yes, and I would still screw the acrylic based on my experience.

Good advice. Mine all did like the second video even with primer 94 and many attempts.

The stuff is nowhere near as good as the Internet would have you believe. Not for this application.

It’s great for many things but not really for larger plastic windows that expand and contract.

[Matt Johnson]

Doesn't need primer on the gelcoat, but needs something on the acrylic.

Typically you make a blacked out window frit by abrading the acrylic and using black spray paint. Rust-Oleum is fine for the job. This acts as the base for the VHB to adhere to and stops UV from attacking the tape.

Depend on size, two layers of VHB tape can be stacked to get the necessary thickness for the caulk. I use it more to hold the window in place while the DOW 795 cures, then as the primary sealant.

I did probably twenty of these jobs (mostly friends) between Florida and Guatemala 5-8 years ago and I have never had anyone contact me since saying they had leaks.

[s/v Jedi]

Originally Posted by Chotu

Good advice. Mine all did like the second video even with primer 94 and many attempts.

The stuff is nowhere near as good as the Internet would have you believe. Not for this application.

It’s great for many things but not really for larger plastic windows that expand and contract.

Like Matt also wrote: yours failed because the tape was too thin to cooe with the heat expansion. You can buy thicker tape and also pit two layers on top of each other to gain thickness.

I have 3’ long windows that require only one layer, but I believe you have 8’ ling windows!

[Chotu]

Originally Posted by s/v Jedi

Like Matt also wrote: yours failed because the tape was too thin to cooe with the heat expansion. You can buy thicker tape and also pit two layers on top of each other to gain thickness.

I have 3’ long windows that require only one layer, but I believe you have 8’ ling windows!

3M engineering/support decided which tape I should use (the thickest made), how to apply it and to use primer 94.

Didn’t work even on the 44” windows.

It held them. They didn’t fall off. But they looked exactly like the windows in the second video. Partially detached.

[s/v Jedi]

Originally Posted by Chotu

3M engineering decided which tape I should use. (The thickest made)

Didn’t work even on the 44” windows.

They should have recommended you better. Before I started with this I had the experience with Sikaflex which has instructions to calculate bead thickness precisely for their products and the same methodology works for all other brands and adhesives too.

[Matt Johnson]

I still think your issue was also very thin polycarbonate. It's extremely difficult to keep a seal on anything that flexes as much as your (3/16"?) Polycarbonate without a frame or mechanical fasteners.

[Tupaia]

Originally Posted by Matt Johnson

Doesn't need primer on the gelcoat, but needs something on the acrylic.

Typically you make a blacked out window frit by abrading the acrylic and using black spray paint. Rust-Oleum is fine for the job. This acts as the base for the VHB to adhere to and stops UV from attacking the tape.

Depend on size, two layers of VHB tape can be stacked to get the necessary thickness for the caulk. I use it more to hold the window in place while the DOW 795 cures, then as the primary sealant.

I did probably twenty of these jobs (mostly friends) between Florida and Guatemala 5-8 years ago and I have never had anyone contact me since saying they had leaks.

I didn't use any tape or primer. But the method is the same. My boat has 12 wrap around windows. Leaked many times until did this. Abrade Acrylic and gel coat frame. Paint a frame on the Acrylic to limit UV and hide the gunge. Clean, clean and clean again. I used SG4000 GE equivalent of DOW 795. Apply it evenly to both surfaces. Thickness has to be sufficient to allow expansion and contraction. If windows abut then use foam beading strip to ensure only 2 surfaces are joined, not 3 as movement causes 3rd surface connection to shear. Do not use any mechanical fixings or screws as this will crack the Acrylic.

[Chotu]

Originally Posted by Matt Johnson

I still think your issue was also very thin polycarbonate. It's extremely difficult to keep a seal on anything that flexes as much as your (3/16"?) Polycarbonate without a frame or mechanical fasteners.

1/4” but also polycarbonate. Just thankful that I moved to an actual mechanical very small frames. World of difference.

[Matt Johnson]

Originally Posted by Chotu

1/4” but also polycarbonate. Just thankful that I moved to an actual mechanical very small frames. World of difference.

I think you found a great solution in the end. It really the fun of boat building. Isn't it?

[Chotu]

Originally Posted by Matt Johnson

I think you found a great solution in the end. It really the fun of boat building. Isn't it?

truly. I loved spending tens and tens of hours maybe even hundreds doing my windows over and over and over again.

There is no emoji to convey the emotion ha ha