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Window replacement

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Joined: 24 August 2002
Location: Montreal, QC
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    Posted: 29 December 2020 at 11:12pm
Here's a run down of the window replacement I undertook a few years ago on Decompression.

The windows are perpetual sources of leaks, this prompted me to investigate using a different attachment method with a view to solving the leaks. This lead me to using double sided adhesive tape and adhesive sealant without using any fasteners. This method of installation has proven to be watertight over a numbers of years of racing the boat.

I chose Dow Corning 795 as the sealant, this is a UV stable structural adhesive sealant. It has very strong adhesion and flexibility, ideal for this application.

For the tape, I used 3M VHB, a high-strength, double-sided acrylic foam tape.

The windows are acrylic (also known also as Plexiglass) but with one important change from the original windows. The new windows are 3/4 inch larger on each side. The reason for this is for adhesion purposes, where the original windows were sealed the gelcoat has a lot of porosity making it impossible to get a clean surface. By making the window larger the sealant will be adhered to a surface that never had sealant on it. The 3M VHB tape will stick sufficiently to this area, it's purpose is to hold the window while applying the Dow Corning 795, space out the window to allow sufficient thickness of sealant that will allow for movement without shearing off, as a filler behind the sealant and for cosmetic reasons.

I had a local shop make the windows using the original windows as a starting point for the template. The corners are all the same radius as original. The outside edge around the window should have a radius routered and preferably flamed around the edge after all machining. Leave the masking in place until you're ready to install.

Once your old windows are removed and all cleaned up, reinstall the windows temporarily with a few fasteners to hold it in place. At this point I stuck a few blocks of wood in place on the deck using a removable 3/4 inch shim between the block and the old windows. The blocks will be used to locate the new window for installation. I used the same 3M VHB tape on the blocks of wood.

The photos below show the blocks and 3M VHB in place ready for the new windows.

Once the new windows are in place and adhered to the 3M VHB tape you can remove the locating blocks. Use masking tape and paper as needed around the window and on the outside of the window to allow for easier cleanup after applying the Dow Corning 795 as this can be a challenge to fill the space, you may need a spatula to help push in the sealant. I used black sealant so taping is important for the cleanup.

On the inside I used 1/8 inch thick plastic angles to make new mouldings. One side was cut down to fit the horizontal surface, I made allowances as I used the same 3M VHB on the vertical surfaces to stick the mouldings in place. The corners were mitered and glued.

Looking back I should have blacked out the edge of the moulding against the window as the white can be seen from the outside.

Here's a photo where you can see the difference in windows on Decompression versus the original windows.

I'm very happy with the result, no leaks and it looks very clean. The image quality is low in some cases as the boat was under a tarp.

Paul White
CAN 214
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