Short answer
Most windows and hatches in leisure boats are made of acrylic (Plexiglas, PMMA) or polycarbonate (PC). A window that leaks or has cracked should be replaced: remove the old one, clean the frame, prepare the new window and frame, apply a sealant made for plastic glazing and fit the window. Minor scratches can often be sanded and polished out.
Repair, polish or replace?
| Problem | What to do |
|---|---|
| Light scratches, dull surface | Sand and polish, see below |
| Leaking at the edge | Remove the window and reseal |
| Cracks from screw holes or corners | Replace the window |
| Window is yellowed or full of small cracks | Replace the window |
| Large windscreen or load-bearing window | Have a boatyard or specialist do the job |
What you need
| You need | For |
|---|---|
| New window, cut to match the old one | Bring the old window as a template |
| Sealant for plastic glazing, e.g. Sikaflex-295 UV | Bonding and sealing the window |
| Cleaner and primer as per Sika's table | Getting the sealant to bond |
| 120 grit sandpaper | Removing scratch-resistant coating where the window is bonded |
| Spacers and masking tape | Even joint thickness, clean edge |
| Plastic scraper, thin wire or cable | Cutting the old window free |
| Nitrile gloves and safety glasses | Protection from sealant and primer |
Replacing the window step by step
- Remove the old window. Unscrew trim strips and screws. Cut through the joint with a thin wire. Pry gently with a plastic scraper.
- Clean the frame. Scrape off all old sealant. Surfaces must be clean, dry and free of grease, oil and dust.
- Prepare the window. If the window has a scratch-resistant coating, sand it off where the adhesive will bond, using 120-grit sandpaper. Then follow Sika's pre-treatment chart for PMMA or PC.
- Protect the joint from sunlight. Sunlight passing through the window breaks down the joint. Sika states that the adhesive joint behind a transparent window must be protected from UV, for example with a black border or an opaque layer.
- Apply the sealant. Sikaflex-295 UV is applied as a triangular bead so the joint has an even thickness. Use spacers so the window isn't pressed down against the frame.
- Fit the window straight away. The window must be in place within the open time, which is about 45 minutes at 23 °C and shorter in warm, humid weather. Never fit the window into sealant that has already skinned over.
- Tool and let cure. Tool the joint within the skin time, about 60 minutes. Remove the tape. Leave the window undisturbed until the sealant has cured.
Sikaflex-295 UV can be used between 10 and 35 °C. 15 to 25 °C is best.
Cleaning and removing scratches
- Wash with lukewarm water, a little washing-up liquid and a soft cloth.
- Do not use products containing benzene, ethanol, alcohol or thinners. They can damage the acrylic. Isopropyl alcohol is fine.
- Sand scratches in acrylic with 240-grit wet sandpaper, then 400 or 600 grit. Then polish with acrylic polish or ordinary car polish.
- Ask the manufacturer before sanding polycarbonate or coated windows. If you sand off the coating, the window becomes less durable.
Common mistakes
- Silicone or ordinary building sealant. It lets go and leaves a film that stops new sealant from bonding.
- Overtightened screws. Acrylic cracks easily from the screw holes.
- No UV protection for the joint behind a clear window.
- Working in the cold or in direct sun.
Safety
Sealant and primer contain substances that can irritate skin and airways. Read the safety data sheet, wear gloves and ventilate. Never clean your skin with solvents. Sika recommends cleaning wipes or hand cleaner and water.
Frequently asked questions
Is it acrylic or polycarbonate in my boat?
Acrylic (Plexiglas) is most common in windows and hatches. Polycarbonate is tougher and used where impacts are a risk. If the material isn't marked on the pane, ask the boat's builder or the seller of the pane.
Can I use Sikaflex-291i for the window?
Sika has a dedicated product for plastic glazing, Sikaflex-295 UV. It withstands weather and sun better and is made for bonding windows.
How do I remove scratches from the window?
On acrylic: wet sand with 240 grit, then 400 or 600, and polish. The manufacturer of PLEXIGLAS describes this method.
Can I clean the window with glass cleaner?
Use lukewarm water with a little washing-up liquid. Avoid products containing alcohol or solvents. Isopropyl alcohol is fine on acrylic.
Sources
- Sikaflex-295 UV, produktdatablad, Sika industry.sika.com
- Pre-treatment chart for sealing and bonding in marine applications, Sika industry.sika.com
- Cleaning, maintaining and disinfecting PLEXIGLAS, Röhm/PLEXIGLAS plexiglas.de