Help! What Sealant is Better than Dow 795 for polycarbonate??

[mdenize]

I can also take some photos if this would help

[Chotu]

Was also considering epoxying a piece of u-channel along the top to simply direct water over the gap and onto the window where it runs off.

Any thoughts about that?

That's the sole purpose of the sealant here. Not to hold the window in place (the VHB tape that's unseen inside does that). Just need to redirect water into the face of the window skipping the crack where it meets the surface.

Maybe z channel?

[Chotu]

Like this?

[Chotu]

Or translated into house terms, a bit of black flashing would work. Seem viable? Then I could do this only once and never need to redo it.

The windows are stuck on like crazy. You'd need a grinder to remove them. The VHB tape is working extremely well. No hope of ever getting them back off. So... I'm just needing to redirect water onto the face of the window past the gap along the top edge.

Or roofing tape or roofing tar??

[PegasusIX]

To solve your problem, create a rebate in your polycarbonate, so the sealant width is much greater than the thickness of the VHB tape. Then add a closed cell polyurethane strip against the poly carbonate, so your sealant is 2:1 ratio perpendicular to you cabin sides, ie 7mm x 3mm, with the 7mm dimension directly out from the cabin sides. In this way you have a thin flexible sealant bead, that will be able to deform with the temperature movement, without building up the stress that overcomes the adhesive bond. Another solution that hay help is to cut the widows in half in the longest dimension, and join with a rebate that overlaps, and then seal the join with sealant. That way the amount of movement the sealant needs to accomodate is halved. Biggest mistake made is to assume sealant the same thickness as the VHB tape will accomodate the thermal movement. It won’t. If you are scientifically inclined calculate the differential thermal movement between fibreglass and polycarbonate, from your local low overnight temperature to the high of black polycarbonate in sunlight on a hot day. I used 50 deg C for the polycarbonate in sunlight on a 35 deg C day.

[PegasusIX]

Best sealant to use in my experience is the Fixtec for Acrylic/plolycarbonate sealing. Follow 3M recommendations for cleaning base with alcohol and wipe in one direction prior to installing VHB and sealant. VHB needs to be compressed to 100psi or something, use special roller to achieve.

[PegasusIX]

Best sealant shape is 2:1 rectangular. And if this is across the direction of movement it will allow deformation in an “S” shape.

[Franziska]

Sikaflex 295uv is a sealant/glue specifically formulated to glue artificial glass panels on boats.
It has UV resistance built in.
It needs a special primer and a special black undercoat when used properly. Both are also available from Sika.
They also have very precise specifications and informations as to how to use the product for best results.
I reglued windows on my boat as well on clients boats. Works great if done properly.

Sika 296 is the correct sealant and glue for real glass windows in marine applications. It also needs the correct primer and workflow.

https://usa.sika.com

They have Pdf files with instructions on their site for both.

[Chotu]

I'm leaning away from the goop at this point. Seems like a less than ideal way to keep water out of an 1/8" crack that moves a lot.

Seems a physical barrier would be better.

Some sort of flashing would work better and never need recaulking.

I can't uninstall the windows (VHB tape), so the ideas of altering the mating surfaces aren't possible.

[Bill whitmore]

GE 1200 series silicone is the best choice

[Chotu]

In order to solve the problem once and for all and not have to experiment, I got myself some black roof flashing tape. That is bridging the gap. Directing the water from the roof down onto the window pain. Looks like it should be good. I put a test patch out there today. It doesn’t attack poly carbonate.

[skipgundlach]

Originally Posted by Chotu

Yep. I'm hitting close to 100% elongation. That's the problem. Good catch.

Also a good point about the tints (and paint I used to frame the windows to hide the bonding like cars do) being very hot.

I'll take a look at 3M 4000 UV and make sure it's compatible with polycarbonate and give it a shot.

DON'T use it without calling and talking to the head tech there.

Unless they've solved their formulation issues, you'll be redoing it shortly, while it fails along the way.

A VERY long thread on the subject I authored includes details of my issues; my solution would not work for you, as it was teak-to-fiberglass. But the failure had nothing to do with the contacted materials.

A snippet, from that thread:
An extensive conversation, at long last after chasing each other around missing each other, with 3M's head tech guy, Todd Jessen, established:

My preparation (clip not-applicable-to-you) was as good as it gets

Application was perfect (ditto)

Cure time and environment was appropriate.

After which, he admitted that they had had a "formulation problem" with
4000UV that they were still trying to sort out.

(clip more not-your-case)

But the fact that he told me not to go back with new 4000UV, free or not,
that they were still working out kinks in the formulation, says volumes.

(end snippet)

I've never looked back. But if YOU decide you want to give it a try, call 3M and ask for him - and see if they have the formulation right, yet.

[Chotu]

Originally Posted by skipgundlach

DON'T use it without calling and talking to the head tech there.

Unless they've solved their formulation issues, you'll be redoing it shortly, while it fails along the way.

A VERY long thread on the subject I authored includes details of my issues; my solution would not work for you, as it was teak-to-fiberglass. But the failure had nothing to do with the contacted materials.

A snippet, from that thread:
An extensive conversation, at long last after chasing each other around missing each other, with 3M's head tech guy, Todd Jessen, established:

My preparation (clip not-applicable-to-you) was as good as it gets

Application was perfect (ditto)

Cure time and environment was appropriate.

After which, he admitted that they had had a "formulation problem" with
4000UV that they were still trying to sort out.

(clip more not-your-case)

But the fact that he told me not to go back with new 4000UV, free or not,
that they were still working out kinks in the formulation, says volumes.

(end snippet)

I've never looked back. But if YOU decide you want to give it a try, call 3M and ask for him - and see if they have the formulation right, yet.

Thank you Very much for that first hand experience with the product. That’s exactly why I chose not to go with any type of sealant. They all are terrible. Nothing really works well in less you have the perfect rebate in the hole. I have two flat surfaces. They are not going to bond. Except with VHB tape, which is holding them perfectly. I just don’t want any drips of water getting down even to the VHB tape. So I am using black roofing Flashing tape. It directs the water from the roof down onto the window pane. Once it is on the window pain, it can’t get in the boat at all. Goes right to the deck.

[joelhemington]

Originally Posted by Chotu

Yep. I'm hitting close to 100% elongation. That's the problem. Good catch.

Also a good point about the tints (and paint I used to frame the windows to hide the bonding like cars do) being very hot.

I'll take a look at 3M 4000 UV and make sure it's compatible with polycarbonate and give it a shot.

Note: Glass also moves due to heat. Assuming this is flat glass (curved would be cost prohibitive) I'd opt for tempered glass (which is always custom made as it can't be cut after the tempering process) with a 3M P18 window film on the inside for heat control. This stuff makes tinted glass look like your weak little sister in terms of heat control, and it will hold the glass together should you break it until you can get it replaced. The only downside to tempered glass is that it's very fragile on the edges. Tinted glass does not repel that much heat but will cause the glass to move even more due to the fact that it will absorb more heat than clear glass. With the tempered glass installed, any good exterior grade silicon would probably do the trick.

[Franziska]

Sorry, but I do strongly disagree.

Never use silicone for this. Any trained boat builder will tell you this.
Silicone has only very very few uses on a boat.

If you ever want to repaint a surface with a tiny bit of silicone on it you will know why. If you get just a spec of silicone on the sandpaper while surface prepping it will be nicely distributed and the result will be pinholes on the new paint job. A lot of them. Even with copious amounts of silicone remover after sanding it's extremely hard to avoid.

Polyurethane or acrylic based sealant do not suffer from this.

Originally Posted by Bill whitmore

GE 1200 series silicone is the best choice