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Plexiglass window thickness?

Sep 30, 2013
2,881
1988 Catalina 22 central Florida
I'm in the process of replacing the leaky old tempered glass windows in our Albin Vega with dark smoked Plexiglass. The plan is to use the two-side tape and Dow795 method, no fasteners. The windows have no curvature, and the openings are about 7" tall by 48" long.

Does anyone have any industry inside information, strength data, or hard-earned personal experience to help me determine the appropriate thickness to use?
 
Mar 20, 2012
3,983
Cal 34-III, MacGregor 25 Salem, Oregon
there is lots of information is available online... search acrylic strength properties. here is the plexiglas page.
with the install method you are using, go with 3/8. it will be stronger than the original 1/4" stuff...
I will eventually do this to our Cal, but I plan to use 1/2" plexi and add mechanical fasteners to insure it stays in place in the event of troubles offshore...
 
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
Gene:

I think you will find that the factory drilled holes in the acrylic and used fasteners to secure the windows while the 795 cures (I think it is about a week or so). Then the fasteners are removed and the holes are filled with 795.
 
Nov 6, 2006
8,443
Hunter 34 Mandeville Louisiana
Dunno, Scott.. the OEM ones in my 34 are 1/8" and never had a problem except they are now getting shabby looking and I will replace with 3/8". The originals have been whacked with breaking waves which caused no problems.. I was surprised when I found that they were so thin.. like I said, I will be going back thicker when I change them.
 
Sep 8, 2014
2,549
Catalina 22 Swing Keel San Diego
Gene,
I just did this on my C22. I believe I have 3/16" but I will check to make sure when I can get home and measure a scrap piece. The next size available was 1/4". I would have preferred the thicker material as I thought the thinner would be too flimsy. Not so, once cut out and installed they are plenty strong enough and the thinner glass is much cheaper than the 1/4".

The 3M tape method is excellent, it adhere extremely well to the glass and the boat. Keep in mind, don't do what I did... I wasn't confident in using black paint to black out the glass so you can't see the tape through. Instead I used a razor to apply a thin layer of 795 as the blackout instead of paint. Bad idea... the 3M tape does not stick to it very well. At the end of the day the 3M stuck like glue to the painted hull, but not so well to the cured 795. I ended using just a couple screws to make up the difference and backed them out later, then filled the tiny holes with 795. Use paint, the pros do and it works.
The only draw back to this method is that its kind of pain to caulk the 795 into the gap on the outside and then on the inside. You have to do a few hours work taping everything off cause it gets messy. I'm pretty satisfied with how it came out, I am 100% confident my weather seal is bomb proof, but I will alter my technique if I ever do it again to make a neater 'finished' look of the bead. I would probably use a large 600CC syringe to apply the 795 in the hard to reach areas.... I dunno.
 
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Sep 30, 2013
2,881
1988 Catalina 22 central Florida
Well, that's ANOTHER thing I was wondering about: how visible the gray tape would be through the smoked Plexi. I had been optimistically hoping I could just roll with it and be okay. You would say not? Cuz the paint thing worries me too. It would basically double the number of surfaces to adhere, thus doubling the chances of failure.

I wonder about maybe sanding the area to be adhered. That would certainly result in greatly reduced see-through ...?

The good thing is, I have four small windows (3.5" x 13") to practice on, so I should be a pro by the time I get to the big ones. :)
 
Mar 20, 2012
3,983
Cal 34-III, MacGregor 25 Salem, Oregon
Cuz the paint thing worries me too. It would basically double the number of surfaces to adhere, thus doubling the chances of failure.

I wonder about maybe sanding the area to be adhered. That would certainly result in greatly reduced see-through ...?
s. :)
im not sure why the paint thing is so worrisome for people.... IF you are using the correct paint.
make sure its a paint formulated for painting plastics.

dont peel the protective paper from the sheeting until you are ready to stick the finished window to the window hole....
after cutting out the new window lens and with the paper still in place, hold the newly cut lens piece it up to the hole, in its proper place, and trace around the hole with a pen or pencil onto the paper.....
then lay the lens down and use a razor blade to gently cut the traced line... then peel the paper off around the edge that will be painted, leaving the paper in the center... if there is any over spray on the remaining paper, it will be peeled off later anyway...

using a scotchbrite pad to scuff the area that is going to be painted is a good way to insure better adhesion...practice is to use a paint made for painting plastics.
all the sanding and beveling of the edges can be done before removing the paper, but the flame polishing must be done AFTER removing the paper... if not it can catch fire and scorch the plastic. at the very least it will be very difficult to get the melted paper adhesive off of the acrylic...

so, cut to lens size.... bevel and sand edges.... trace hole in cabin on to lens paper..... cut trace line, peel edge paper.... scuff and paint, let dry very well... flame polish..... add adhesive tape and install.
 
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Jul 30, 2013
1
Morgan 24 and 34 Columbus
Hi Gene, I did a plexi replacement on my '68 Morgan 34, ...they look great! Except where i did'nt back paint prior to install. The dow didnt fill entirely and you can see all clearly thru my smoked bronze plexi.
 
Sep 8, 2014
2,549
Catalina 22 Swing Keel San Diego
Gene,
Once you cut out your windows you should have plenty of scrap plexi to test paint adhesion and the 3M tape bond. Sorry for my late response, I was in Grad school this weekend and with the new squeeze (she's smoking hot and very needy, sorry). I have some scrap plexi and will attempt this test at home and post for all. I think gloss black Rustoleum will do the trick, no sanding required. We shall see...
 
Jun 19, 2013
855
Oday 28 Traverse City
I used 1/4", as that was what is available. Painted the back, then 3M VHB tape, and a little caouling with Dow795. Turned out great.
I bought a roll of the 3M VHB tape, so have enough for another window replacement if interested.
 
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Sep 30, 2013
2,881
1988 Catalina 22 central Florida
Gene,
Once you cut out your windows you should have plenty of scrap plexi to test paint adhesion and the 3M tape bond. Sorry for my late response, I was in Grad school this weekend and with the new squeeze (she's smoking hot and very needy, sorry). I have some scrap plexi and will attempt this test at home and post for all. I think gloss black Rustoleum will do the trick, no sanding required. We shall see...
I'm not painting.

It took two days of searching and looking and googling and asking around and reading and researching to come to the conclusion that no one could give me any straight answers as to a specific paint to use. In the final analysis, I would prefer to have some hull showing through the window, than for the window to fall off the cabin without any warning and go kerplunk in the middle of the ocean. No test I perform here and now could possibly be a reliable indicator of what will happen five years from now and thousands of miles away.

Smokin' hot and needy, huh? Damn, I miss being young and single sometimes. :biggrin:
 
Sep 30, 2013
2,881
1988 Catalina 22 central Florida
I used 1/4", as that was what is available. Painted the back, then 3M VHB tape, and a little caouling with Dow795. Turned out great.
I bought a roll of the 3M VHB tape, so have enough for another window replacement if interested.
I'm going with 1/4" also. The process has begun. :)

 
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Jan 22, 2008
1,465
Hunter 34 Alameda CA
Gene, I somehow missed this discussion, but offer the following:
The paint I used was a single component urethane from Interlux called Brightside. If you look at my photo, you can see that I spaced the paint such that my 3/8" butyl windshield bedding just came up to the paint, but not over for maximum adhesion. I rolled that part where you can still see the white showing a little before continuing. I did roughen the gel coat up a bit with a Scotchbrite Pad in that entire area within the blue tape. Then, you can see the blue tape is spaced exactly the width and height of the Acrylic, so the 795 will only adhere to the gel coat and the acrylic and butt up to the butyl (down in about 1/4"). Since I only had one chance to get it right, I mounted some wooden registration blocks at the edge of the masking tape. That way I started with it aligned and pushed it down compressing my butyl to about 1/2 its thickness. That stuff is so sticky, I don't think it would ever let go of the window again. It will never leak past that for sure. I was left with a 3/16" standoff gap that I went back and filled with the 795. I tooled the perimeter with a gloved hand wetted with mineral spirits. Then pull the tape off both the cabintop and the Acrylic while it's still wet. I beleive I have maximum adhesion by avoiding the painted surface with its risk of lifting off. I think you will like the results with no fasteners. Five years....my port side is now 5 years and no sign of the 795 separating from either the acylic or the cabintop. :)

Allan

Butyl Tape in Place.JPG
 
Aug 3, 2012
2,517
Performance Cruising Telstar 28 302 Watkins Glen
I replaced my portlights with plexi, and I tried taping some in place to avoid drilling holes. It worked well, even with 1/2" plexi. I held the glued portlight in place while my wife taped it. 48 hours later, I removed the tape. No problems so far!
 
Sep 30, 2013
2,881
1988 Catalina 22 central Florida
I'd like to thank everyone who has been helping ... the windows are coming along! All four of the small, forward windows are are done, as is one of the large main saloon windows. I was ready to start the last one when the weather turned cold, and I'm waiting for a warmer spell to to finish. I hope to have the project done Sunday (86 degrees forecasted).

The hardest part about this, by far, is masking the doghouse around the window. It needs to be PERFECT, as this Dow 795 is extremely unforgiving. The job is not as clean as I'd like, but the imperfections are not noticeable until you're approaching arm's length away.





 
Sep 8, 2014
2,549
Catalina 22 Swing Keel San Diego
Looks outstanding Gene, and I'm sure the extra thickness is worth every penny. How does the tape look up close?