Step by Step: How to Replace Your Boat Windows
- Telicia
- Dec 17, 2025
- 7 min read
From handover, we had problems with the acrylic windows on our Leopard 45 Catamaran. They were leaking, letting seawater and rainwater ingress into the hull, an issue that was unfortunately widespread across the range.
In the end, every single window on the boat was pulled off and replaced, either with glass or Perspex. In this article, I'll share why our boat windows leaked, the step-by-step process for installed new boat windows, and some feedback on switching from plexiglass to actual glass in the saloon.
CONTENTS:
Issues with Leopard Catamaran windows
For those who are curious, here's a breakdown of everything that went wrong with our windows, and the solution to stop the leaks. For a more detailed look, you can also check out our YouTube episode that explores the topic.
Improper bonding
On a number of windows we found gaps in the sealing bond line, providing a path for water to come inside. In many, foam had been stuffed around the outside of this bond line, which is not meant to be there according to the SOP and Robertson & Caine staff. This means it was added in the factory, against procedure, presumably to help stop water leaking through improperly bonded windows. This in turn allows the window to pass early leak tests, but means the window will leak when the foam fails or is overcome by water.
The solution to this problem is obvious; make sure you have a solid bead of sealant all the way around the bond line that meets the product specifications.
Window frames
Removing our windows revealed that a number of the frames were surrounded by gaps that led into the hull of the boat, or were cracking and breaking out of the hull. Water was able to ingress through those gaps and cracks, explaining the source of certain leaks we knew were present but struggled to find.
To fix this properly, the frames were essentially rebuilt using structural materials like fiberglass and West Systems 404 filler.
UV protection separating
The procedure used to install Perspex windows on the Leopard Catamaran sail series, including the Leopard 45, Leopard 42 and Leopard 50, is quite complicated. It involves various products including a screen printer ink that's applied to the window as a UV barrier. The problem occurs when this screen printer ink starts flaking off the Perspex... is screen printer ink appropriate for a structural bond on a marine window?
Our solution was to use a different bonding procedure which removed the need for a separately applied UV barrier, which I'll outline below.
How we rebonded our catamaran windows
Rather than use Robertson & Caine's procedure, we chose to follow the advice of other Leopard owners and use Dow 795 instead. It's a one-part neutral curing adhesive used to bond skyscraper windows in, and has had very positive feedback from many of the boat owners who'd been down this path before us.
Dow 795 doesn't require a primer and is highly resistant to UV, weather and temperature changes. We followed the application information in the technical data sheet, and used 20 oz sausages in a Makita caulking tool with a wide nozzle to bond the windows. For the backfill, we used narrow nozzles or a piece of plastic hose attached to a nozzle, depending on the size of the gap we needed to backfill.
Products used to fit the boat windows
This a complete list of links to the products used to redo our catamaran windows. You will notice the 3M fastener and blocks; this is a way of bracing the windows while they cure, that doesn't require elaborate structures or drilling holes in the hull.
Preparation
Fitment
Bracing
Wooden blocks
Wooden wedges
Process used to rebed our yacht windows
Here's a basic overview of the process used to replace our boat windows.
Preparing to rebond the windows
1. Start by removing the original acrylic windows. To remove ours, long, thin blades and sharpened scrapers were used to cut the sealant. Silicone spray was regularly applied to lubricate the blades and reduce the effort needed to cut.
You want to side load the sealant as soon as possible using air wedges (our go to choice), soft wooden wedges or someone pulling on the leading edge, as many sealants (including the Sikaflex 295 UV that originally bonded our windows) have less cut resistance when under side load.
2. After taking the windows out, remove any primer and sealant left behind on the frame and window.
3. Once the window frame is completely clean, it needs to be prepared for re-bonding. Our boat's frames have a gel coat finish, so to prepare for re-bonding the gel coat was keyed up with P220 sandpaper (As per the Dow 795 requirements), then scrubbed down with acetone. A final wipe of isopropyl was done just before bonding.
4. The windows also need to be prepared for bonding.
Marine glass windows should just be cleaned with something that does not leave a residue, like 99% Isopropyl.
New marine acrylic windows, such as Perspex or Plexiglass, should have the bonding surface keyed up with 80 grit sandpaper by hand and then wiped clean with 99% Isopropyl. Make sure only the bonding surface is keyed up, not anything that's visible when you look through the window. Blue painters tape is useful for marking this out.
Acrylic windows that are being re-used need to have all of the original sealant and any other contaminants removed, then prepared for bonding the same way new acrylic windows are. Avoid acetone, it will damage the plastic.
5. Whatever you've chosen to braced the windows should be prepared so they're ready when you need them.
6. We wanted a uniform gap all the way around so a set number of pop sticks were taped together and then taped to the deck for the window to sit on top of while it cured. Some were parallel to the window and some perpendicular.
7. Prior to fitting the windows, the Dow 795 was kept inside the air conditioned saloon where the air temperature was cooler. The window fitment was planned for one of the coolest parts of the day as we were in hot, tropical climate at the time. These two actions gave us more time to work before the sealant started to skin over and cure.
Bonding the windows
8. Apply the sealant to the window frame, ensuring you're aware of the sealant's tolerance to heat and humidity, and any specific instructions for the surface preparation and application.
We used Dow 795 sealant in an electric caulking gun with a thick caulking tip for the sides and bottom, and bent caulking tip for the top edge. A V-shape was cut into the side of the nozzle to give the necessary height.
Having done one window with a manual sausage/canister gun, we found it applied adhesive too slowly for the Trinidadian heat, which is why electric was used.
One solid bead of sealant was laid down around the entire window frame, then addition lines were added to hold in extended or irregularly shaped parts of the window against the hull.
9. Once the sealant had been applied to the frame, the window itself was cleaned and checked one last time before being put in place. It's best to wear gloves to handle the clean new window so grease from fingers does not contaminate the bond line.
When the window was being pushing into place, one person was on the inside of the boat using spacers made of pop sticks taped together to ensure the gap between the window and frame was the right size - you can check the recommended bead height in the sealant TDS.
When the gap was correct, the pop sticks were taped in place so that the window couldn't come closer and the gap remained at the right distance. How big that gap was, was determined by the sealant and the type of window - glass experiences less thermal expansion than Plexiglass or Perspex.
While the person was monitoring the gap on the inside, two people were outside, adjusting the placement of the window then adding their choice of braces to ensure everything stayed in the correct position while the sealant cured.
Keep in mind that pushing a Perspex window into a curve will require more pressure than a pre-curved glass window or a straight Perspex window. While the Leopard 45 originally comes with Perspex windows bonded into a curve, we chose to either have curved glass or straight Perspex, as we preferred not to have the natural shape of the Perspex fighting to pull away from the bond.
10. After leaving the windows to cure for the amount of time required by the sealant, the retaining system is removed and the final backfill done. To ensure a neat edge to the backfill we use silicone shaping tools.
Feedback on switching to marine glass windows
Something we decided to do during this process was change all of the saloon windows and the helm skylight to toughened marine glass, while the rest of the boat remains the OEM Perspex.
It's GREAT, we have no regrets!
Those windows are much clearer, easier to clean .They're also tinted, offering excellent UV protection in the saloon - this was an issue we had with the Perspex, which offers no UV protection.
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