Gelcoat Blistering: What It Is, Why It Happens, and What To Do

When You First Notice It

It’s the start of a new boating season. You walk up to your boat in the yard, freshly cleaned, sitting on blocks. At first, everything looks great.

Then you notice it. Small bumps scattered across the hull — like the surface has broken out in a rash. It’s not “boat chicken pox.” It’s gelcoat blistering.

What Is Gelcoat Blistering?

Blistering appears as raised bubbles or pockmarks on the surface of your hull, most commonly below the waterline. These blisters form when:

• Water penetrates beneath the gelcoat
• Pressure builds between layers
• The gelcoat separates from the underlying fiberglass

Why Do Blisters Form?

While there are several contributing factors, most cases trace back to material quality and manufacturing defects. Low-quality, water-permeable resins are the most common root cause — a construction issue, not a maintenance failure. Too much hardener in the gelcoat or fiberglass, improper wetting of the mat during layup, and voids in the structure all create weak points where water can get in and separation can begin.

Why Blistering Gets Worse

Blisters are not just cosmetic. Left untreated, water continues to migrate through the laminate, additional blisters form, and the underlying fiberglass may degrade. In severe cases, large sections of the hull can be affected and repairs become significantly more expensive.

Repair Options

The correct approach depends on the severity and extent of the blistering. For a few isolated blisters, we grind each one down to solid fiberglass, rebuild, and refinish. For widespread blistering, the only reliable long-term solution is a full structural repair: remove the gelcoat, grind down to sound fiberglass, rebuild the laminate as needed, and reapply a gelcoat or barrier system. A cheaper cosmetic option (sand, fair, and repaint the bottom) improves appearance but does not solve the root problem — and the blistering usually returns.

Final Takeaway

Gelcoat blistering is typically a symptom of deeper structural or material issues, not just surface damage. Your approach should depend on the extent of blistering, how long you plan to keep the boat, and your tolerance for ongoing maintenance versus upfront cost. For minor cases, spot repair may be enough. For widespread blistering, a full structural repair is the only reliable long-term fix.