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Restoring windows and portlights — seals and acrylic

Repairing leaking windows and portlights: replacing seals, re-bedding and restoring hazed acrylic.

A sailing yacht heeling in the wind

Leaking windows and portlights are one of the commonest causes of damp inside and — worse — of a soaked coachroof core. Over time sealants lose their elasticity, acrylic goes dull and cracks, and frames corrode. Restoring them brings back watertightness and clarity and prevents expensive structural repairs. The best way to diagnose a leak is to hose the window down while watching from inside, though the water can travel a long way from where it gets in.

Author: Maciej Baniewicz · Boat & Yard editorial team

Methodology: The material was prepared from the sources given and from practical service rules; the parameters have to be confirmed in the manual for the specific manufacturer or model.

Boat windows fall into those set in frames (aluminium or steel) and bonded frameless ones, where the acrylic pane sits directly against the coachroof. On framed windows the leak usually comes from a degraded rubber seal or from the sealing between the frame and the skin. On bonded windows the problem is aged adhesive sealant. The repair method depends on the construction, so first establish which type you have.

Restoring a framed window means taking it out, cleaning the grooves of the remains of old rubber and sealant, and fitting a new seal of the right profile. Clean corrosion off the frame and paint it if necessary. When refitting, apply a flexible sealant between the frame and the skin, and tighten the screws evenly without squeezing the joint down to nothing — the sealant needs thickness in order to work. Remove the excess before it cures.

Acrylic (Plexiglas, PMMA) or polycarbonate panes are replaced when they are cracked or badly scratched. Cut the acrylic to size with an allowance, drilling the holes oversize to allow for thermal movement (acrylic moves a great deal with temperature changes) and never tightening the screws rigidly, or the material will crack. For bonding frameless panes, use flexible structural adhesives made for the purpose (Sika or an equivalent), absolutely compatible with acrylic — some sealants cause stress crazing in the plastic.

Dulled but sound Plexiglas can be saved by polishing instead of replacing. Wet sand the surface with increasingly fine papers (from about 400 to 2000), then polish with acrylic compound to bring back the clarity. Light scratches come out with the polish alone. After the restoration it is worth applying a protective product with a UV filter, because it is UV that causes acrylic to dull and become brittle. Regular washing with a soft cloth (never dry) extends the life of the panes.

Choosing the materials is crucial for durability. Sealants must be compatible with acrylic and with the substrate (gelcoat, aluminium) and resistant to UV and seawater; where bonding is needed, use structural products, and where only watertightness matters, flexible sealants. Avoid silicones anywhere that will later be painted or bonded, because they contaminate the surface. Always check the data sheet for compatibility with acrylic, so as not to cause cracking.

Safety and practice: when sanding acrylic, wear a mask and goggles, and when working with adhesives and solvents ensure ventilation and protect your skin. When removing windows, support heavy panes so they cannot fall, and work without haste, because acrylic is easy to scratch or stress. After the repair, wait before launching or a water test until the sealant has fully cured (sometimes several days). Patience while the compound cures decides whether it stays watertight for years.

Material prices — products and options

Indicative prices in EUR (incl. VAT), as of July 2026. Products widely available in Europe; prices depend on the shop, the pack size and consumption. The links lead to example shops that ship across the EU. Dollar amounts are converted at 1 EUR = 1.1622 USD (ECB, 7 September 2026) and rounded — treat them as a rough guide, not a price in a US shop. Imperial values in brackets are conversions of the metric figure.

Rubber seal of the right profile
  • GEBO — seals for boat windows
    original rubber profiles for Gebo portlights and windows, various sizes (per metre / set)    €10–40 (≈$12–46)
  • Universal EPDM profile
    rubber U/H-profile seal for window frames, resistant to UV and salt water (per metre)    €3–8 (≈$3–9)
Acrylic (PMMA) or polycarbonate pane cut to size
Flexible sealant / structural adhesive compatible with acrylic
  • Sika — Sikaflex 295 UV
    adhesive sealant made for bonding PMMA/PC windows, UV resistant, 300 ml (10 fl oz)    €26–35 (≈$30–41)
  • Sika — Sikaflex 292i
    single-pack structural adhesive for dynamically loaded joints, 300 ml (10 fl oz)    €17–25 (≈$20–29)
  • Sika — Sikaflex 291i
    all-purpose adhesive sealant for portlight frames and seals, 300 ml (10 fl oz)    €8–15 (≈$9–17)
Acrylic polish and wet-and-dry papers
Protective product with a UV filter for Plexiglas
Plastic-compatible degreaser and preparation
Fixings
Ancillary materials and safety gear

Recommended materials and sources