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Bonding Acyrlic port lights to a painted boat - good idea?


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I'm helping someone out with a cabin window / port light replacement project, their frames are completely shot & basically scrap so they're going with acrylic directly onto the side of the coachroof.

The modern way seems to be to bond them on using a modern structural silicone or 3m VHB tape, which sounds great, no fastener holes.
However the boat has been painted with a 2 part acrylic urethane topcoat, so the windows would be bonding not onto the original gelcoat that was laid up with the hull, but the paint.

Is this an issue? I have no idea of the bond strength of the paint to topsides.
If so I guess we could sand it back to gel in way of the window bonding area & the bead of sealant around the outside should cover the edge of it making it impossible to see once the window is fitted.

 

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If the paint is well adhered and in good shape, scuff it up and install the lights. If there's any doubt, take it back to gelcoat. any pictures?

as to the technique, mine are mounted to gelcoat but I used the VHB tape method with a sealant for the bead and it worked out really well. I used dow corning 795.

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If I had any concerns about the paint I would test it with a tape pull or pulling on some VHB tape.  On my van I installed rear door windows using 3M sealant onto the paint after cleaning and slight scuffing.  That was what the factory does and what the supplier of the windows said to do.  

 

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Hmm no worries about how well the paint is adhered, but I'm tempted to go back to the gelcoat it is. No pics unfortunately.

I guess with vans they need to bond onto paint anyway to stop it rusting, plus if the window falls out the side of a van its an inconvenience (well maybe not for the guy driving behind), at sea it could be a lot worse.

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I'd say gelcoat and paint adhering is similar but you are worried about nothing.  Let's say the bond is 1" overlap. Window is say 18" long x 4" high. Total perimeter area is 2(18+4) x 1" = 44 square inches. Even if the bond is only good for 200 psi, thats 8800 lbs of pressure to rip it off.

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2 hours ago, Zonker said:

I'd say gelcoat and paint adhering is similar but you are worried about nothing.  Let's say the bond is 1" overlap. Window is say 18" long x 4" high. Total perimeter area is 2(18+4) x 1" = 44 square inches. Even if the bond is only good for 200 psi, thats 8800 lbs of pressure to rip it off.

That'd be to pull it off all at once. More likely, it would peel up from a corner or other weak point so the area that needs to fail at any one time would be much smaller. Even so, I would be pretty comfortable bonding it without fasteners as long as there wasn't a lot of pre-load from the window needing to be bent to conform to the shape of the base. 

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UHB tape directly to sound paint is best. UHB tape prefers the smooth surface of paint over some roughed up composite. Not so sure that scuffed paint is better than glossy clean two-part... Leave a few mm space between the UHB and the exterior edge into which sealant is injected. Silicone is likely the best. If two-part peels off you have bigger problems.

Also important: Paint the bonding surface of the Acrylic window, usually black. This prevents the sunlight from degrading the UHB tape bond. I used top-shelf rattle-can spray paint primer. Careful masking makes it look sharp inside and out. One of my portlights is well curved. Holds just fine. UHB takes a while to develop full strength. Press the parts together a few times over a few days ... I use my feet to really push hard.

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Well the decision has been made for me, we're going onto the paint.
I'm sure it will be fine, its not an offshore boat, my main concern was the relying on one bond from gelcoat to paint & then a second from paint to acrylic, but it shouldn't see too much abuse.

Some helpful tips, thanks.

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19 hours ago, El Boracho said:

Also important: Paint the bonding surface of the Acrylic window, usually black. This prevents the sunlight from degrading the UHB tape bond.

That's called "fritting" isn't it?

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15 hours ago, Max Rockatansky said:

Only you don’t paint the inside where the bond is, but the outside...

I'd heard that most people paint the inside using a suitable primer, as it will basically look a bit naff on the outside.
But I guess we're adding quite a lot of layers that all need to adhere well at this point.

I guess the other option is stainless fasteners with oversize holes to try and prevent cracking, but they never look as good.

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I posted at this site pics and descriptions of my salon window re-installation, so you are welcome to search

I made damn sure the glue faces were as clean as possible, so no paint. I was careful to space the windows to the specs in the adhesive (Dow 795) instructions. I had painted a fritting, as was previously done, on the outside perimeter of the outside face.

it has now been four years this fall and looking good. These are big (4x2 foot)

BTW no fasteners, never had them. I reinstalled at 24 years, and I think it probably was the first time, although there was evidence of halfassed gooing up

The reason for spacing is to give enough thickness so that differential expansion won’t shear the glue. It’s the same reason for oversized fastener holes, but instead of drilling a bunch of leak and fracture risks, just.space.

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5 hours ago, MiddayGun said:

I guess the other option is stainless fasteners with oversize holes to try and prevent cracking, but they never look as good.

please don't do that.

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My windows were bigger - about 2.5' x 5.5'  (vertical inside mullion doesn't show through).

They never leaked and took the odd big wave.

For scale, the hatches adjacent are 24" x 24" (27"x27" outside dimensions)

small%201.jpg 

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  • 3 months later...

I've got a similar problem on my H-Boat.

Cabin house sides painted with Awlgrip. Acrylic port lights. The lights are mounted with SS wood screws that go through the acrylic and house side, and into a teak strip that runs along the edge of the opening. There is some black Boat Life silicone sealant between the light and the cabin house. There are a few screws that go from the inside of the teak strip into the cabin house.

On the Port side, the openings in the cabin house are a little too tall, maybe 1/4", although the lights are the same P&S. The pics below don't show this but it's evident. You can kinda see in pic #2 that the screws on the bottom edge aft are pretty close to the edge.

During heavy rain I get some leaking P&S.

The first pic is just the openings, no lights. The others are after installing the lights. There are way more outside mounting screws than inside ones.

The boat is going to the shop for a spa treatment, and I'd like to get these port lights fixed. I like the look of the outside screws and the inside teak trim strips, but I suspect there is a better way to do it.

I welcome your advice.

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On 9/29/2020 at 4:56 AM, MiddayGun said:

I'm helping someone out with a cabin window / port light replacement project, their frames are completely shot & basically scrap so they're going with acrylic directly onto the side of the coachroof.

The modern way seems to be to bond them on using a modern structural silicone or 3m VHB tape, which sounds great, no fastener holes.
However the boat has been painted with a 2 part acrylic urethane topcoat, so the windows would be bonding not onto the original gelcoat that was laid up with the hull, but the paint.

Is this an issue? I have no idea of the bond strength of the paint to topsides.
If so I guess we could sand it back to gel in way of the window bonding area & the bead of sealant around the outside should cover the edge of it making it impossible to see once the window is fitted.

 

Follow the instructions 

https://usa.sika.com/content/dam/dms/us01/d/Bonding and Sealing Plastic Windows.pdf

 

vhb tape is a poor choice , to achieve bond strength tape  requires clamping pressure 

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I think he hasn't.  I had exactly the same setup on my Dash 34 @Bull City, and I decided there was no way to keep the fasteners and stop the leaks with acrylic windows.  They expand and contract so much that they break the bond on any sealant around a fastener.

I had new windows made without the screw holes and did mine with VHB tape.  The install was easy - the results were great, but not perfect, because I had a few small low areas on the window edge that I didn't fill.  The tape stuck initially then lifted after 6 months or so in two small areas.  Had I prepared the surface properly to make it flat, this would not have happened.  5-ish years later when I sold the boat there are no additional leaks over the minor ones that developed due to inadequate prep on my part. 

I think there are two good choices, both of which involve ditching the fasteners.

1) If the window edge is quite flat already, consider the vhb tape solution.  The tape is not terribly cheap - I had to buy a 100' roll for around $120 CAD.  I used about 40' and sold the rest.  Once a few people saw the results with this method they wanted to redo their windows the same way. 

I put the VHB tape around the edges, stuck the windows on (you get one chance only - think contact cement) then created a tape edge and used black sealant (silicone?  Lifecaulk? Can't remember) to fill the remaining gap on the outer edges.  If I ever had to do it again it would be super easy to clean up, but frankly I am not sure the vhb will let go without some application of heat or cutting the tape out with a wire.

You would need to make sure you did not make the mistake I made.  Make sure the sealing edge is flat and fair.

VHB tape does come in various thicknesses - if you just have a little waviness in the edge just order a thicker tape, but be careful that the windows don't end up being proud on the frame.  Scratches and gaps under a straight edge will need to be filled.

To apply the inward pressure I borrowed a few suction cups from a friend, stuck them to the windows on the inside, tied them together with rope and weighted the rope.  This worked very well.  In my case the window frames are curved while the acrylic windows are not.  I wondered if the VHB would hold them in the curve.  That tape is amazing stuff - once I had the windows on I realized they were there forever.

2) As @Ishmael suggests, use the 795 adhesive.  Messy, but likely the right solution if your window edges are quite rough.  The adhesive will fill the gaps.  You will need to apply pressure inwards while it cures - see the suction cup method above.

Don't try the Sikka - I tried it and it was a disaster.  I ended up scraping it off after 6 months and trying again with Lifecaulk.  That was just as messy and an equal disaster.

Best advice I can offer.  These are the only two solutions I have heard DIY'er have success with.

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56 minutes ago, Will1073 said:

Have you ever used VHB tape?

I've had a VHB failure too, on a window with a slight curvature. Mine were clamped with a vacuum bag for 24 hours with proper surfaces and prep. It's not happy with standing loads. No-one ever mentions the clean-up issues with VHB tape either.

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13 minutes ago, Russell Brown said:

I've had a VHB failure too, on a window with a slight curvature. Mine were clamped with a vacuum bag for 24 hours with proper surfaces and prep. It's not happy with standing loads. No-one ever mentions the clean-up issues with VHB tape either.

That's because there aren't any?  Am I misunderstanding something?  Do you mean removing it if you have to re-do it?

My curved window frames with vhb tape held my acrylic windows on extremely well.  It might be a problem with very thick acrylic on a curved frame. 

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I'll add another vote for the 795 camp. You'll be able to keep almost the same look, minus the screws and leaks

Might need some creativity, but if you plan ahead and mask appropriately it's not too messy. I went through 8 tubes of the stuff on my recent port project and am really happy with how it turned out.

Note the spacers to set the glue line thickness - it's critical that you don't squeeze too much out. I used a 1/8" gap on the small ports and 1/4" on the longer ones. Mine have a painted faux-frame on the outside to maintain the look of the boat, but yours wouldn't need that. You do want some "fritting" (matte black spray paint) on the back of the acrylic, in the glued area, to protect the glue from UV, and to give a cleaner look. Sand all the bonding surfaces with 80 grit.

PXL_20201229_232008024.jpg

PXL_20201228_220104809.jpg

PXL_20201228_220039264.jpg

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8 hours ago, Will1073 said:

Have you ever used VHB tape?

Yes 

and I have read the application instructions 

“ Bond strength is dependent upon the amount of adhesive-to- surface contact developed. Firm application pressure develops better adhesive contact and helps improve bond strength. (Steps C and D) Generally, this means that the tape should experience at least 15 psi (100 kPa) in roll down or platen pressure “

 

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6 hours ago, andykane said:

I'll add another vote for the 795 camp. You'll be able to keep almost the same look, minus the screws and leaks

Might need some creativity, but if you plan ahead and mask appropriately it's not too messy. I went through 8 tubes of the stuff on my recent port project and am really happy with how it turned out.

Note the spacers to set the glue line thickness - it's critical that you don't squeeze too much out. I used a 1/8" gap on the small ports and 1/4" on the longer ones. Mine have a painted faux-frame on the outside to maintain the look of the boat, but yours wouldn't need that. You do want some "fritting" (matte black spray paint) on the back of the acrylic, in the glued area, to protect the glue from UV, and to give a cleaner look. Sand all the bonding surfaces with 80 grit.

PXL_20201229_232008024.jpg

PXL_20201228_220104809.jpg

PXL_20201228_220039264.jpg

In future ,  to develop bond link thickness with sika or 3m , use these clear rubber bumper dots

they come in many different sizes 

 

A0F5146C-410E-4E94-9D28-B67273D904E5.jpeg

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12 hours ago, Russell Brown said:

I've had a VHB failure too, on a window with a slight curvature. Mine were clamped with a vacuum bag for 24 hours with proper surfaces and prep. It's not happy with standing loads. No-one ever mentions the clean-up issues with VHB tape either.

Yah 

you must mold the window to the frame  or the installation will pull apart, dis bond  , over time 

The other  failure point with window installations  is UV burn of the bond line 

 

this bond line must be shielded from UV 

several methods available 

google FRIT

 

 

 

 

3FB44580-9133-4CA1-BE61-784EB84AD560.jpeg

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4 hours ago, slug zitski said:

In future ,  to develop bond link thickness with sika or 3m , use these clear rubber bumper dots

they come in many different sizes 

I'm sure those would work too, but strangely I couldn't find any in my pile of scraps.

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1 minute ago, andykane said:

I'm sure those would work too, but strangely I couldn't find any in my pile of scraps.

Handy to have an assortment of different size bumpers in your tool tray 

they have many applications 

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12 hours ago, andykane said:

I'll add another vote for the 795 camp. You'll be able to keep almost the same look, minus the screws and leaks

Andy, a couple of questions. 

  1. How did you do the clamping?
  2. On the outside of the windows, is that a white protective film?

Thanks.

12 hours ago, Rain Man said:

I decided there was no way to keep the fasteners and stop the leaks with acrylic windows.  They expand and contract so much that they break the bond on any sealant around a fastener.

From Rain Man and others, it sounds like fasteners are a no-no with these methods, VHB or 795, so I'll probably need to get new windows. Mine are original, 1979 or 1980, and are in very good shape and as I recall they're bendy along their length and easily conformed to the curvature of the deck house. What material is best? Acrylic?

Or I could snip off the heads of the screws and set them in the counter sinks with 795. That would preserve the look (although I might spend the rest of my days re-gluing them when they fall out).

As I recall, the deck house surface under the window is very smooth. We used Boat Life Life Seal sealant (see NOTE). Maybe the issue is the fasteners and/or not having left a thick enough amount between surfaces.

Thanks for the advice & suggestions. The popular options seem to be VHB or 795. Please add anything else that comes to mind. It's going to be several weeks before I get to this. The boat will be in the shop where the 2015 renovation was done, and I'm going to have the yard guy do the work - I know I would make a mess of this.

 

NOTE: From spec sheet: "A unique combination of marine silicone and polyurethane, formulated especially for fiberglass... Although LifeSeal® Fast Cure was specifically designed for fiberglass and vinyl, it adheres tenaciously to metal, glass, wood, and other types of boat building materials. It provides a durable, permanent, watertight seal for all properly designed joints subject to structural movement."

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4 minutes ago, Bull City said:

@Rain Man Thanks.

Also saw this regarding sealants on their  website:

https://hatchmasters.com/which-sealant-to-use/

"The only three adhesives I would consider using are Sika Flex 295 UV with the primer, GE SG-4000, and Dow 795."

You won't regret losing the fasteners.  Your boat will look fantastic without them.  It was a big improvement on mine.

You will have to decide whether or not to keep the teak strips on the inside.  I ditched mine.  I think it made the interior feel a little more spacious.

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2 hours ago, Bull City said:

Or I could snip off the heads of the screws and set them in the counter sinks with 795.

Do not countersink fasteners in acrylic - it will very likely crack.

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You don't need real clamping force for install since the 795 has a bit of stiction when the window is pressed into the thick bed of silicone.  You will need the support blocks on the bottom (andy's second pic) to keep them from sliding down because of gravity.  The blocks can be double sided taped to the cabin side and easily removed after install.  Then touch up the bottom edge after the blocks are removed.  Careful masking makes cleanup simple and silicone edges sharp. 

Like you said your windows were probably pulled too tight to the cabin sides during install squeezing most of the silicone from the joint.  And the joint failed from expansion and contraction.  That being said - I would guess you could keep your windows and use the screws to set the thickness and just hold the window in position up against a much thicker layer (1/8" or more) of 795.  It should work because many boats including yours were done this way.  Removing all the old silicone and prepping the surfaces is very important.

As others have said new windows with no fasteners do look awesome.  But then you need to make/buy new ones.

The thought of trying to remove VHB tape in the future scares me.  I envision damage just like what happens with 5200.  

PS - great looking boat.

 

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1 hour ago, yoyo said:

The thought of trying to remove VHB tape in the future scares me.  I envision damage just like what happens with 5200.  

They make it look easy, but then, of course they do.

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Dang - they sure did make it look easy.   I envisioned myself seriously messing up paint or gel coat trying to get it off the sides of a cabin when pulling a window - maybe not.     

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2 minutes ago, yoyo said:

Dang - they sure did make it look easy.   I envisioned myself seriously messing up paint or gel coat trying to get it off the sides of a cabin when pulling a window - maybe not.     

Well, you would need to buy or rent the vibrating tool and buy some of the adhesive removal spray, but it sure looks a lot easier that scraping off 795-like adhesive.  I did that three times on my boat trying to get the windows with fasteners to seal (I tried Sika, butyl and Lifecaulk) before I had success with VHB tape.

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10 hours ago, Bull City said:
  1. How did you do the clamping?
  2. On the outside of the windows, is that a white protective film?

I used a strip of masking tape, then used hot melt glue to stick a couple support blocks below the window. On the smaller flat windows this is all it takes - the 795 is goopy enough you can just push it into place and it'll stay as it cures.

For the longer ports, there was a very slight curve (maybe 1/2" over the length). For these I used the same kind of support blocks to get the height/alignment right, then those cross pieces on the inside to keep it pulled tight along the curve. I again used hot melt glue to stick some blocks to the protective paper so I had something to pull on the inside.

Yes, there's a protective film on the acrylic (it comes from the factory that way). The white is spray paint because mine have painted "faux frames" to match the original look but yours wouldn't need this.

PXL_20210109_180934809.jpg

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11 hours ago, andykane said:

I used a strip of masking tape, then used hot melt glue to stick a couple support blocks below the window. On the smaller flat windows this is all it takes - the 795 is goopy enough you can just push it into place and it'll stay as it cures.

For the longer ports, there was a very slight curve (maybe 1/2" over the length). For these I used the same kind of support blocks to get the height/alignment right, then those cross pieces on the inside to keep it pulled tight along the curve. I again used hot melt glue to stick some blocks to the protective paper so I had something to pull on the inside.

Yes, there's a protective film on the acrylic (it comes from the factory that way). The white is spray paint because mine have painted "faux frames" to match the original look but yours wouldn't need this

To be sure I understand, you put a strip of masking tape (Is blue tape OK?) on the inside of the window, and then hot-glue a piece of wood that you can clamp to the cross piece. Very clever!

How long since you did it, and how is it holding up?

 

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2 hours ago, Bull City said:

To be sure I understand, you put a strip of masking tape (Is blue tape OK?) on the inside of the window, and then hot-glue a piece of wood that you can clamp to the cross piece. Very clever!

How long since you did it, and how is it holding up?

I glued straight to the protective film, but tape should work too. 

I only just did this so can't personally comment on longevity, but I followed the same process as @Zonker (his wife wrote a great article about it), and I think that lasted them for a circumnavigation. My understanding is it should last as long as the acrylic.

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7 hours ago, andykane said:

I glued straight to the protective film, but tape should work too. 

So you must have removed some of the film, leaving enough for the glue.

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7 hours ago, andykane said:

I only just did this so can't personally comment on longevity, but I followed the same process as @Zonker (his wife wrote a great article about it), and I think that lasted them for a circumnavigation. My understanding is it should last as long as the acrylic.

Maybe @Zonker could share a link to the article if it's on line??

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1 hour ago, Bull City said:

So you must have removed some of the film, leaving enough for the glue.

Yeah - it was removed when I painted the black fritting on the inside.

Here's a photo showing the film trimmed for 1" fritting, right before painting. You can see it's sanded to give some tooth for the paint and glue to stick.

PXL_20201215_005054560.jpg

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On 1/24/2021 at 2:29 AM, Bull City said:

From Rain Man and others, it sounds like fasteners are a no-no with these methods, VHB or 795, so I'll probably need to get new windows. Mine are original, 1979 or 1980, and are in very good shape and as I recall they're bendy along their length and easily conformed to the curvature of the deck house. What material is best? Acrylic?

Or I could snip off the heads of the screws and set them in the counter sinks with 795. That would preserve the look (although I might spend the rest of my days re-gluing them when they fall out).

If your windows are still OK, you could do what we did when rebedding the front windows on our catamaran, which is fill the holes in the existing windows with thickened epoxy and paint the fritting on the outside so the filled holes don't show. The holes in the boat were filled with the Sika as the windows went on. We were doing big windows so left a couple of the holes unfilled until right at the end of the job to use to locate and lightly clamp the window in place with a couple of the old fasteners. Once set, we took them out and filled the remaining holes. 

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As for clamping/hold, note the wood battens in my above pics, screwed to holes were already there, likely from the initial install. Easy enough to fill before installing the perimeter bead/fillet, or use the goo to fill

IMO, bonding to paint (substrate or window) means you are dependent on how well the paint adheres. YMMV

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7 hours ago, Max Rockatansky said:

As for clamping/hold, note the wood battens in my above pics, screwed to holes were already there, likely from the initial install. Easy enough to fill before installing the perimeter bead/fillet, or use the goo to fill

IMO, bonding to paint (substrate or window) means you are dependent on how well the paint adheres. YMMV

For this reason, I didn't do any fritting on my windows when I used the VHB tape because I didn't know what paint would adhere well to the acrylic given its high thermal expansion coefficient.  Instead, I had the windows made from very dark grey acrylic, and you could just barely see the tape through the window.  Up here in the PNW UV isn't that big a deal, but down south fritting is probably the way to go.

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3 hours ago, Rain Man said:

For this reason, I didn't do any fritting on my windows when I used the VHB tape because I didn't know what paint would adhere well to the acrylic given its high thermal expansion coefficient.  Instead, I had the windows made from very dark grey acrylic, and you could just barely see the tape through the window.  Up here in the PNW UV isn't that big a deal, but down south fritting is probably the way to go.

Use Sika 209d , a black solvent based polyurethane. 

apply with foam brush . One coat on the inside , bonding area,  of the window 

extend the  frit about 10 percent  past the flange bonding zone to shade the adhesive 

 

 

 

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2651DA75-64C2-42AB-A223-AD5CEFE4B2B3.jpeg

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  • 5 months later...

This has been very helpful. My boat has been in the yard getting a new electric pod drive installed, and I added replacing port lights to the list. I discussed it with the guy who is doing the work, and he was spot on the problem I had: too many fasteners, they fit the holes too snugly, and there was not enough sealant. The large port lights had 27 fasteners each, now they have 6 each. The small ones had about 11, now 4. The holes are oversized, and the overlap edges are fritted.

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On 1/22/2021 at 5:27 PM, slug zitski said:

Follow the instructions 

https://usa.sika.com/content/dam/dms/us01/d/Bonding and Sealing Plastic Windows.pdf

vhb tape is a poor choice , to achieve bond strength tape  requires clamping pressure 

VHB does not require clamping pressure to get bond strength.  It does require initial 15 pounds minimum application pressure which can be achieved with a hand J roller to activate the acrylic adhesive.  VHB is a good choice made even better if used with Dow 795.  The critical part is preparation of substrates. 

Sika fails in hot climates.  Sika has far less ability to resist expansion/ contraction forces at bond line vs Dow795.

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On 1/23/2021 at 8:53 PM, Rain Man said:

They make it look easy, but then, of course they do.

If Brent Bystrom says it, you can trust it.  I personally know him and he's considered one of the top application engineers in 3M.

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4 hours ago, rksailsolo said:

VHB does not require clamping pressure to get bond strength.  It does require initial 15 pounds minimum application pressure which can be achieved with a hand J roller to activate the acrylic adhesive.  VHB is a good choice made even better if used with Dow 795.  The critical part is preparation of substrates. 

Sika fails in hot climates.  Sika has far less ability to resist expansion/ contraction forces at bond line vs Dow795.

I have no idea what you are trying to say 

VHR requires clamping pressure 

Sika require no clamping pressure 

I have seen properly installed windows last for 20 years when bonded with Sika 

bond line thickness is in relation to substrate , window movement 

follow the instructions 

 

if your application allows clamping pressure then be my guest… use tape 

 

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2 hours ago, slug zitski said:

I have no idea what you are trying to say 

VHR requires clamping pressure 

Sika require no clamping pressure 

I have seen properly installed windows last for 20 years when bonded with Sika 

bond line thickness is in relation to substrate , window movement 

follow the instructions 

 

if your application allows clamping pressure then be my guest… use tape 

 

You have some idea....clamping vs application difference is keeping two substrates under constant pressure throughout the cure (clamp) vs ensuring those two substrates are pressed into the VHB with minimum 15 pounds of pressure during initial joining of the two substrates (application).

You will note in my later post on bracing that it was for the sealant.

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11 hours ago, rksailsolo said:

VHB does not require clamping pressure to get bond strength.  It does require initial 15 pounds minimum application pressure which can be achieved with a hand J roller to activate the acrylic adhesive.  VHB is a good choice made even better if used with Dow 795.  The critical part is preparation of substrates. 

Sika fails in hot climates.  Sika has far less ability to resist expansion/ contraction forces at bond line vs Dow795.

this is EXACTLY how I did the fixed ports on my columbia. VHB and a wooden roller. actually on the port window, I didn't have a wooden roller so I just kept going around the frame with a rag and my hand pressure. then I taped off the acrylic and the cabin and filled the gaps with 795, smoothed in with a gloved finger. 4 years later neither side leaks and they still look good.

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