Gelcoat Defects and What Causes Them: Fisheyes, Pinholes, Pre-Release and the Rest

Why this guide exists

When a moulding comes out of the mould with a surface fault, the first instinct is often to suspect the material. Sometimes that is right. Far more often, the composites troubleshooting literature points somewhere else: to mould preparation, to how thickly and evenly the film went on, to the state of cure when lamination started, or to workshop conditions on the day.

That matters commercially as well as technically. A fabricator who changes gelcoat supplier over a fault caused by residual release agent will meet the same fault with the new supplier. This page sets out what the published literature attributes each common fault to, so that the diagnosis starts in the right place.

It is a buyer-level orientation, not a repair procedure. Nothing here tells you how to fix a part, and nothing here is a dosage, a film thickness or a cure schedule. Those belong to the manufacturer’s documentation for the gelcoat in front of you.

Faults that point at the mould

Fisheyes, sometimes called de-wetting, appear as small craters where the gelcoat has pulled away from the mould face. The accepted explanation is a surface-tension mismatch, most often caused by contamination: oil, silicone from an unrelated product used nearby, or too much release agent or residue left behind. The diagnosis is usually in the mould preparation rather than in the drum.

Pre-release is the related mould-side fault, where the cured film separates from the mould before the part is ready, leaving dull or distorted patches. Sources attribute it to uneven or excessive film thickness, an unusually fast cure, or lamination that was delayed too long after the gelcoat gelled.

The practical reading of both: mould condition and release-agent discipline are part of the gelcoat system, not a separate concern from it.

Faults that point at film thickness

Excessive or uneven thickness is the single most frequently cited cause across the literature, and it produces several different symptoms. Cracking and crazing are attributed to too much thickness, to rapid temperature change, or to stress at high-load areas of a part with an inadequate laminate behind it. Sagging on a vertical face is attributed to a heavy film combined with high spray pressure, low viscosity or a long gel time.

This is where the intuition that more gelcoat means more protection does real damage. The gelcoat layer has no reinforcement in it. Making it thicker adds material that has to carry its own cure stress without any fibre to distribute it, and the result is a layer more likely to crack, not a layer with more life in it.

The useful target is an even film of the thickness the manufacturer specifies for that grade, applied consistently across the part.

Faults that point at timing and cure

Alligatoring, the wrinkled pattern that looks like alligator skin, is generally attributed to laminating over a gelcoat film that was not yet cured enough to take it, or to uneven cure caused by inconsistent thickness, catalyst level or temperature. The gelcoat needs to reach a particular state before the laminate goes on, and both too early and too late cause identifiable problems.

Production blistering, as distinct from in-service blistering, is generally linked to trapped air or moisture and incomplete cure beneath the surface. Colour variation and patchiness are attributed to pigment dispersion and mixing practice, and to batch-to-batch consistency in the supplied material.

The common thread is that a gelcoat is not finished when it has been applied. It is finished when it has reached the right cure state and the laminate has gone on at the right moment, and both of those depend on conditions in the workshop on the day.

What weathers later, and why it is a different conversation

Some things buyers call defects are not production faults at all. Chalking is the resin binder at the surface breaking down under prolonged sunlight into a powdery residue. Fading and gradual loss of gloss follow the same mechanism. Fibre prominence, where the reinforcement begins to show through, happens because resin erodes faster than glass.

These are weathering behaviours described across the industry rather than manufacturing faults, and they are the reason gelcoat grades intended for outdoor or immersed service exist at all. If a part is dulling after years outdoors, the question to ask is whether the gelcoat specified was the right one for that exposure, not whether the batch was defective.

In-service blistering on immersed parts belongs in the same category. Marine literature describes it as hydrolysis of the ester linkages over long water contact and reports it as more common with orthophthalic chemistry, which is precisely the argument for an isophthalic gelcoat on parts that stay wet.

What we can and cannot do about a fault

Samrat FRP Traders is a trader, supplier, importer and reseller of FRP materials in Nepal. We supply gelcoat manufactured by Samrat Poly Resins, India. We do not formulate it, test it, inspect mouldings or issue technical determinations about a production problem.

What we can do is put a specific, well-described question to the supplier, and supply the documentation they publish for the grade you are using. A useful description covers the part and the mould, the conditions on the day, how the gelcoat was applied and how long after gelling the laminate went on. A photograph on its own rarely settles anything.

Where a grade’s supplier documentation does not answer the question, we will tell you that plainly rather than answer from a general article.

Frequently asked questions