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

The result is a surface that looks—and behaves—compromised.

Why Do Blisters Form?

While there are several contributing factors, most cases trace back to material quality and manufacturing defects.

Primary Causes

1. Low-Quality Resins

The most common root issue.

• Lower-grade resins are more water-permeable
• Over time, water migrates through the hull
• This leads to separation and blister formation

This is a construction issue, not a maintenance failure.

2. Too Much Hardener in Gelcoat

Excess hardener causes:

• Excessive heat during curing
• Premature blistering—even in the mold

These defects often appear early in the boat’s life.

3. Too Much Hardener in Fiberglass

When over-hardened:

• Fiberglass can become brittle
• Structural integrity is reduced
• Movement and flexing lead to cracking or blistering in the gelcoat

4. Improper Wetting of Fiberglass

During layup, fiberglass mat must be fully saturated.

If not:

• Dry pockets remain
• Bonding between layers is weak or nonexistent
• Gelcoat sits on an unstable foundation

This significantly increases the likelihood of blistering later.

5. Voids in the Structure

Voids are small gaps or low spots caused by:

• Inadequate rolling or pressing during layup
• Poor surface preparation (e.g., insufficient sanding)

These areas:

• Prevent proper bonding
• Create weak points
• Allow separation under stress

Voids can occur anywhere on the boat—not just the hull bottom.

Why Blisters Sometimes Appear Later

Blistering doesn’t always show up immediately.

In many cases, it develops over time due to:

• Repeated hull flexing during normal use
• Weak bonding between gelcoat and fiberglass
• Small cracks forming, allowing water intrusion

Once water enters:

• It becomes trapped beneath the surface
• Pressure builds
• A blister forms

Even pressure washing can accelerate this process—but only if the underlying structure is already compromised.

Why Blistering Gets Worse

Blisters are not just cosmetic.

If left untreated:

• Water continues to migrate through the laminate
• Additional blisters form
• Underlying fiberglass may degrade

In severe cases:

• Large sections of the hull can be affected
• Repairs become significantly more expensive

Repair Options

The correct approach depends on the severity and extent of the blistering.

Option 1: Isolated Blisters (Minor Cases)

If only a few blisters are present:

Process:

• Grind each blister down to solid fiberglass
• Rebuild with new fiberglass
• Apply gelcoat and finish

Pros:

• Lower upfront cost
• Targeted repair

Cons:

• Blisters may continue to appear over time
• Underlying systemic issues remain

Option 2: Cosmetic Repair

For widespread blistering where cost is a concern:

Process:

• Sand the entire bottom
• Fill imperfections with fairing compounds
• Apply bottom paint

Pros:

• Improves appearance
• Lower cost than full restoration

Cons:

• Does not solve the root problem
• Blistering may return

Option 3: Full Structural Repair (Best Long-Term Solution)

For severe or widespread blistering:

Process:

• Completely remove gelcoat
• Grind down to sound fiberglass
• Rebuild laminate as needed
• Reapply gelcoat or barrier system

Pros:

• Addresses root cause
• Long-term solution

Cons:

• High cost
• Dependent on material quality and workmanship

Even with proper repair, outcomes depend on:

• Quality of materials used
• Thoroughness of the repair process

Important Considerations

Hidden Damage Risk

If the underlying fiberglass contains:

• Voids
• Dry mat
• Widespread defects

Repairing visible blisters may expose a much larger problem area.

Buying a Boat? Inspect Carefully

Whether new or used:

• Always hire a qualified marine surveyor
• Inspect the hull thoroughly

Be cautious if:

• The hull has fresh bottom paint
• Recent cosmetic work appears to hide prior damage

These can mask underlying blister issues.

Warranty Reality

If blistering appears on a new boat:

• Act immediately while under warranty

Be aware:

• Manufacturers may attempt to limit liability
• Disputes over responsibility are not uncommon

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
• Your tolerance for ongoing maintenance vs. upfront cost

For minor cases, spot repair may be sufficient.

For widespread blistering, a full structural repair is the only reliable long-term solution.