Gelcoat Application: Avoiding Air Bubbles, Pinholes, and Fish-eyes

Gelcoat is the visible skin of every FRP part — and the layer where surface defects cost the most to repair. Air bubbles, pinholes, and fish-eyes account for the majority of cosmetic rejects in marine, wind, and automotive composite production. Each defect has a distinct root cause, and once you can read the surface, you can eliminate them at the source rather than polishing them out downstream.

Air Bubbles: Viscosity and Spray Control

Bubbles form when entrapped air cannot escape before the gelcoat gels. The two biggest levers are viscosity and atomization. Spray viscosity should sit at 1,500–3,000 cP at 25 °C — too thick traps air, too thin causes sagging on vertical surfaces. Verify every batch with a Brookfield viscometer; do not rely on supplier COA alone.

For spray equipment, use a pressure-pot or airless system with a 0.8–1.2 mm tip at 60–80 psi. Hold the gun 40–50 cm from the mold, perpendicular to the surface, with 50% pass overlap. Apply in two coats of 250–300 µm each rather than one heavy 600 µm pass — solvent and entrained air escape between coats. Catalyst (MEKP) at 1.5–2.0% should be metered, not hand-stirred, to avoid whipping air into the resin.

Pinholes: Outgassing and Substrate Wetting

Pinholes are micro-craters left by gas escaping after the surface skinned over. The usual culprits are cold molds, high humidity, and trapped solvent in the release system. Keep mold temperature between 20–25 °C and shop relative humidity below 65%. A mold colder than the gelcoat causes condensation, which boils off during cure and punches holes through the film.

Let each release-agent coat flash off fully — semi-permanent systems need 15–30 minutes between coats and a final 60-minute cure before gelcoat application. Pinholes clustered along laminate ridges usually indicate the backup laminate was rolled too aggressively, pushing styrene vapor through the still-soft gelcoat. Wait for the gelcoat to reach a firm, tack-free state (typically 2–4 hours) before laminating.

Fish-eyes: Contamination Discipline

Fish-eyes are circular dewetting craters caused by silicone, oil, or wax contamination on the mold or in the air. A single airline with a failed oil-water separator can ruin an entire production shift. Install coalescing filters on every compressed-air line, drain receivers daily, and ban silicone sprays anywhere in the gelcoat bay — including WD-40, mold release from neighboring lines, and personal hand creams.

Wipe molds with a clean solvent (acetone or PMC cleaner), never recycled rags. If fish-eyes appear mid-production, stop immediately — they will not self-heal, and continuing only buries the defect under laminate.

Need Help Selecting the Right Gelcoat?

Resinspot supplies ISO, ortho, vinyl ester, and UV-cure gelcoats with full technical specs and low MOQ for sampling. Our application engineers can review your spray setup, mold conditions, and defect photos to recommend the right grade. Contact us for a technical consultation and free sample.