Gelcoat Blister Repair

By Don Casey
Published: June/July 2013

The discovery of hull blisters elicits a mental "oh no!" from most boat owners. But in many cases, the solution is simple and inexpensive.

Illustration of scrubing the surface cleanAll illustrations: ©2013 Mirto Art Studio,

Don't let a few blisters on your hull intimidate you this spring!

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.

No one looking at a bottom with a few blisters can tell you with certainty the cause, the gravity, or the most appropriate treatment. For that reason, a bit of wait-and-see when blisters are first discovered is likely to be the better and often the least costly course, particularly if your boat spends time out of the water.

That doesn't mean you should simply ignore a few blisters. To the contrary, the acid pus they can contain is not doing the laminate any good, no matter what the cause, so blisters should be attended to as soon after discovery as practical. But ignore advice, whether well-meaning or avaricious, that you must immediately strip all the gelcoat off the bottom and replace it with different barrier coat. That might turn out to be exactly the treatment your boat requires, but it also might not. If your boat comes out of the water covered stem to stern with blisters, you're going to need the advice of a professional, preferably one without a conflict of interest. However, blisters develop slowly, laminate damage slower still, so if the bottom exhibits a few blisters, whether in a cluster or scattered, initially treat them individually.

Repairing individual blisters also cures them. The objective of replacing all of the gel-coat is to inoculate the entire bottom against future blistering, but if 99 percent of the original bottom is already resisting blisters, that makes 99 percent of your investment wasted. Individual blister repair is where to start until it becomes clear whether your bottom has pimples or pox.

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Tech Support

Degree Of Difficulty
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Tools and Materials:
  • Chisel
  • Disc sander
  • Water hose and nozzle
  • Scrub brush
  • Scotch-Brite scrubber pads
  • Sanding block
  • Disposable natural-bristle paintbrushes
  • Isopropyl alcohol
  • Freezer bags (or other thick clear plastic)
  • Duct tape
  • Epoxy (West 105 or equivalent)
  • Colloidal silica (West 406 or equivalent)
  • Fiberglass cloth (6-10 ounces for large blister)


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