Gelcoat Blister Repair

Around 40 years ago, the use of fiberglass and resin to build boats revolutionized the marine manufacturing process.

Renowned for high strength, durability and presumed low maintenance, fiberglass was deemed to be almost indestructible but in the early 70s blisters were first noticed and many rumours circulated as to what was causing them.

Many methods of repair were attempted to remove and stop recurrence of what is now commonly known as osmosis. Because the cause was not fully understood most of the early repair methods were unsuccessful in preventing recurrence of the blisters.

A fiberglass hull is composed of two layers. The base layer, or polyester resin-reinforced fiberglass laminate, is covered by gelcoat, a pigmented polyester resin. All fiberglass laminates are semipermeable, meaning they allow water to pass through the outer layers. Osmosis is the process of moisture seeping into the fiberglass laminate, mostly externally through the gelcoat layer and sometimes internally from permanently wet bilge areas.

As moisture migrates into the laminate it fills voids, becomes trapped and creates an acidic blister fluid. Still seeking to diffuse equally throughout the substrate, the water between the laminate and outer surface places pressure on the gelcoat. The resulting blemishes or blisters that form between the lami nate and the gelcoat affect the appearance and performance of the boat. As excessive moisture is the culprit, blistering usually only occurs below the waterline.

When blisters are left unchecked they will progressively extend deeper into the laminate and affect the structural integrity of the hull. The term used to describe the chemical reaction between polyester resin laminates and water is
hydrolysis. Over time, blister fluids attack the resin in the laminate, severing the chemical bond between the resin and laminate. As it progresses, the bottom becomes spongy and delaminates. Unfortunately, this condition is not reversible
but can be repaired once it has started. Any hydrolyzed laminate on a boat must be removed and the bottom relaminated. This usually requires the services of a professional.

Regardless of the quality of construction, osmosis blistering and water absorption into the laminate occurs sooner or later in most boats constructed using standard polyester resin

Variations in resins, catalysts, environmental conditions during construction and workmanship in the manufacturing process all determine when and to what extent gelcoat blisters appear.


Is osmosis preventable? It’s estimated that one in four fiberglass boats will get gelcoat blisters in its lifetime. The obvious answer is yes, there are preventative measures you can take to slow down and delay this occurrence. The first line of defense is your boats gelcoat. Apply a barrier coating to new hulls before commissioning. Other protective measures such as dry storage, frequent spot repair of defects in the gelcoat and additional hull coatings will help slow down and delay the possibility of blisters forming.

Epoxy coatings applied on a sound, dry hull provide a durable, water-resistant barrier; however no system is foolproof. Even epoxy coatings allow some absorption. The key to a successful repair is to remove all of the damaged laminate, thoroughly dry the hull and correctly apply the barrier coats. To reduce the possibility of blisters reoccurring keep the bilge as dry as possible and the boat well ventilated. Install additional vents to eliminate condensation. Solar-powered vents offer an efficient and affordable means of increasing air flow. Finally, check the barrier coat annually. If scraped and dinged, repair and recoat promptly.