If you have ever spotted bubbles forming in the gelcoat below your boat’s waterline, you have encountered hull blisters. Also called osmotic blisters, these bumps form when water penetrates the fiberglass and reacts with materials inside. Left unchecked, they can worsen over time. Here is how to deal with existing blisters and keep new ones from forming.
Understand What Causes Blisters
Hull blisters develop when water works its way through the gelcoat into the fiberglass laminate beneath. There it reacts with soluble compounds, creating an acidic fluid that pushes outward and forms a bubble. Boats kept in the water for long periods are especially prone, since constant immersion gives moisture more time to penetrate.
Inspect and Assess the Damage
Start by hauling the boat and letting the hull dry. Small, scattered surface blisters are mostly cosmetic and manageable, while widespread or deep blistering into the laminate is more serious. Tap and examine the affected areas to gauge how far the problem extends before deciding on a repair approach.
Open, Clean, and Dry Each Blister
To repair a blister, open it up, drain the fluid, and clean the cavity thoroughly with fresh water. This is important, since the fluid inside is acidic and must be flushed away. Then let the area dry completely, which can take time. A properly dried hull is essential for a lasting repair.
Fill and Fair the Repairs
Once dry, fill the cleaned cavities with a marine epoxy filler, building up the layers as needed. Sand the repairs smooth and fair them into the surrounding surface so the hull is even. Take your time here, because a smooth, well-cured repair holds up far better than a rushed one.
Prevent Future Blisters
Prevention is the best strategy. Applying a quality epoxy barrier coat creates a moisture-resistant layer that helps keep water out of the laminate. Storing the boat out of the water when not in use and keeping the hull in good condition also go a long way toward preventing new blisters.
A Final Thought
Hull blisters are common, but they do not have to become a major problem. Address them early, repair them properly, and protect the hull with a barrier coat. For extensive blistering, a professional marine shop can restore the hull safely.