What Are the Causes of Blistering in a Fiberglass Gelcoat?

Many owners of fiberglass boats, attracted to the no-maintenance material popular in the boat manufacturing industry in the 1970s and 1980 were unpleasantly surprised to find a breakout of blisters below the water line on the boat’s hull 10 to 15 years later.


Older fiberglass boats have more incidents of blistering in the gelcoat.Older fiberglass boats have more incidents of blistering in the gelcoat.
The extent and cause of blistering can vary. Examine the blisters closely to properly determine the cause and the best way to proceed with fixing the damage.

Fiberglass panels consist of layers of woven glass reinforcing threads, or laminates, saturated in polyester or epoxy resin. The final outer coating of resin, called gelcoat, is typically around 1/32 of an inch thick and acts as an aesthetic and protective finish. Bumps in the surface of the gelcoat, called blisters, are primarily caused by permeability of the gelcoat or poor bonding in the fiberglass layers. Break open the gelcoat blister to examine the underlying laminate and determine the root of the problem. Wear protective eyewear, as pressure behind the blister can exceed 150 psi, causing acidic liquid to spray out.

Gelcoat Blisters

Blistering of the fiberglass gelcoat is primarily a cosmetic, surface problem. True gelcoat blistering does not affect the underlying laminates and can easily be fixed when only a handful of blisters occur on a boat’s hull. A slightly permeable gelcoat allows water-soluble chemicals within the laminate to slowly pull water in through the outer coating, where it combines with the chemicals to form a liquid with a larger molecular structure that becomes trapped behind the gelcoat. As more water collects behind the coating, internal pressure builds, creating a dome-shaped blister between the gelcoat and the laminate. Treat the blister by grinding away the damaged gelcoat and filling the cavity with epoxy.

Laminate Blisters

Delamination, or blistering of the underlying laminates, signifies a greater, structural problem in the fiberglass panel and requires more extensive repairs to fix. After removing the outer gelcoat, an undamaged laminate should appear dark and translucent when wet and should give a sharp sound when struck. Blistered and damaged laminate reveals white fibers when exposed and sounds dull when struck. Usually caused by improper installation of the laminates during manufacture, laminate blisters are usually no more of a problem than gelcoat blisters if only the first layers are damaged. Damage of deeper layers requires more extensive repair.

Pox Blisters

Pox blistering, a more serious condition, occurs when a systemic break-out of hundreds of small gelcoat blisters covers the fiberglass surface. Pox blisters are more difficult to fix since grinding away and filling the blisters does not cure the problem. The underlying material is saturated throughout with the water-soluble chemical, and the entire area of gelcoat affected must be removed to allow it to dry for several months before applying a new gelcoat. According to "Sea" magazine, using inferior glycol or a fire-retardant chemical in the gelcoat during manufacturing causes most pox blistering.

About the Author

Ann Salter began writing professionally in 2010 and has worked extensively in the fields of art, architecture and design since 2004. Her work has appeared in informative guides on student housing cooperatives and sustainable building alternatives. Other areas of specialty include technology, health, gardening and cooking. Salter holds a Bachelor of Architectural Studies from the University of Waterloo.