Hull Rx: When & How To Repair Gelcoat Blisters

By Don Casey, BoatU.S. Magazine
Most of us react emotionally to the slightest blemish on our hulls, and blisters often announce themselves as lots of blemishes. Those little bumps become a source of worry, even dread. Why my boat? How much time do I have before hull integrity is compromised? If I spring for a “cure,” how many thousands of dollars will it cost? Take a deep breath: Blisters are a symptom, not a disease. It is estimated that one in four fiberglass boats develops hull blisters. Most will never be more than a cosmetic concern, particularly if they’re treated rather than ignored.

Fiberglass hulls blister because the shiny exterior, the gelcoat layer, is not 100-percent impermeable to water. Seeking equilibrium, the water on the face of the gelcoat is drawn to dry voids on the back side – air pockets in the original lay-up or adhesive failures between laminates (particularly between the gelcoat and first layer). This attraction is intensified by loose soluble materials in the laminate, with which the water combines into larger molecules, too large to pass back through the gelcoat. Worse still, the solutions created are acidic, which can attack the resin and release more soluble material that attracts yet more water. The pressure that necessarily builds is released by expanding the volume of the void, pushing the gelcoat into a dome.

Blisters might signal a major defect due to either inferior materials or poor techniques when the hull was built, but absent a blistering history with other boats from the same builder, you should be slow to draw such a dire conclusion.

Blisters also occur because an inattentive worker may have failed to apply the gelcoat uniformly; because fluctuations in the mix from the resin sprayer may have created spots of soft gelcoat; because features in the mold may have induced thinner coverage; because the gelcoat may have been sanded to excess by the owner or yard; because chemical stripper may have been used on the bottom; because contaminants in the water may have attacked the gelcoat; or a dozen other reasons. – Read on