Boat hull blisters are essentially pockets of moisture and acidic fluid that develop under the gelcoat of fiberglass boats, typically below the waterline. They are caused by water penetrating the hull's layers and reacting with the resin, creating pressure and expanding the cavities. Repairing these blisters involves opening them, cleaning the affected area, and filling the void with an epoxy filler. 

What Causes Blisters?
  • Osmosis:
    Water penetrates porous areas of the fiberglass and resin, reacting with the resin to create an acidic fluid that expands under pressure, forming blisters.
  • Water Permeability:
    Polyester resin and gelcoat, while seemingly solid, are somewhat permeable, allowing water to seep in over time.
  • Stress Cracks:
    Stress cracks, especially below the waterline, can create pathways for water to enter and initiate blister formation. 

How to Repair Blisters:
  1. 1. Open the Blister:
    Use a sharp chisel or screwdriver to puncture the blister, allowing the fluid to drain. Wear eye protection, as the fluid can be acidic and pressurized. 

  2. 2. Clean the Area:
    Use a disk grinder to remove the blister and any surrounding damaged laminate, ensuring the depression is wide enough and shallow enough. 

  3. 3. Flush and Dry:
    Flush the open area with water and scrub it clean with a solution of TSP (trisodium phosphate) and water. Allow the area to dry thoroughly. 

  4. 4. Fill with Epoxy:
    Apply a thickened epoxy filler to the cleaned-out cavity, ensuring it is flush with the surrounding hull. 

  5. 5. Sand and Finish:
    Sand the repaired area smooth and apply paint to match the existing hull finish. 

Important Considerations:
  • Early Detection:
    Blisters can be indicative of a larger problem, so it's crucial to address them promptly. 

  • Scope of Repair:
    The extent of the repair may vary depending on the size and severity of the blisters. 

  • Professional Assistance:
    If you are unsure about the extent of the damage or the repair process, seek professional help from a boat repair specialist like us to help you fix this issue.