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Fibreglass has earned its place as one of the most versatile materials in construction, repairs and DIY projects. Lightweight yet remarkably strong, it’s used everywhere - from boat hulls and car bodywork to roofing, ponds and even artistic projects. For beginners, it will feel like a material that can do almost anything, once you understand its full potential.
Part of the appeal lies in its adaptability...
Need a tough waterproof lining? Fibreglass can do it. Want to mould complex curves or reinforce an existing structure? With the right resin and reinforcement, fibreglass offers a level of flexibility that’s hard to beat. Additives, fillers and gelcoats give you even more control over the final look and finished performance, making it a favourite for both professionals and hobbyists alike.
But like any material, fibreglass has its quirks. If you’re new to working with it, you may have already run into a few frustrations, such as sticky surfaces that refuse to cure, bubbles trapped beneath the surface, layers that won’t bond or finishes that look rough instead of smooth. These issues are common and can be disheartening when you’re just starting out.
That’s where this guide comes in.
Instead of trial and error, Fibreglass Supplies will walk through the most frequent problems beginners face with fibreglass - explaining why they happen and how to fix them, with clear, practical steps. Think of this as your troubleshooting companion: easy to follow, reassuring and designed to help you get better results with every fibreglass project...
Air bubbles are one of the most common fibreglass-related issues faced by beginners. They look like tiny white spots or cloudy patches in the laminate, and while they might seem cosmetic, they actually weaken the structure. Bubbles form when air gets trapped between the fibres and resin, often because of rushed application, poor rolling technique, or using resin that’s too thin or runny for the job. Left untreated, bubbles can spread moisture, reduce strength and spoil the fibreglass finish.
How To Fix It:
A sticky, tacky surface is frustrating because it feels like the fibreglass hasn’t set properly. This happens when the resin mix ratio is off, the temperature is too low, or oxygen has interfered with the cure. With polyester resin, the top surface is particularly vulnerable because oxygen blocks the reaction, leaving it sticky even though the layers beneath may be hard.
How To Fix It:
When layers separate, it’s usually due to poor bonding between them - known as delamination. This happens when the surface is glossy, contaminated, or outside of the recoat window, meaning the resin no longer chemically bonds with the next layer. For beginners, it’s often caused by skipping sanding or failing to clean between coats.
How To Fix It:
Known as “print-through,” this visual issue appears when the pattern texture of the woven fibreglass fabric or chopped strand matting shows up on the finished surface of your project. It’s caused by resin shrinkage as it cures or by laying heavy reinforcement directly under a finish coat. It makes the surface look rough, unprofessional, unfinished and unattractive, especially under paint or gelcoat.
How To Fix It:
Cracks and star fractures often show up as fine lines or spiderweb patterns, typically appearing after an impact or when resin-rich areas of the fibreglass laminate are put under stress. While they may start off small and cosmetic, these defects can quickly grow, reducing both the strength and durability of your composite. Left untreated, they allow moisture to penetrate, which can accelerate damage and lead to costly repairs.
In many cases, starburst cracks form because the fibreglass laminate isn’t distributing load evenly, often due to excessive resin, trapped air or poor reinforcement placement.
How To Fix It:
Blisters - often seen on boat hulls - form when water penetrates and seeps through the gelcoat and reacts with residues in the underlying laminate. This creates acidic fluid pockets that swell under pressure and builds pressure beneath the surface, creating raised bubbles or “blisters” that distort the smooth finish of your hull. While it may start as a cosmetic issue, the swelling pockets can spread over time, delaminating the structure and weakening the hull.
How To Fix It:
If your resin mix overheated, hardened too fast, or even cracked, you’ve experienced a problem called “exotherm.” This happens when resin cures too quickly and generates excessive heat, which can make the laminate brittle, distorted or structurally weak. Exotherm is especially common among beginners who accidentally mix too much resin in one batch or use too high a catalyst ratio.
The larger the volume of resin in the pot, the more heat it builds up internally, accelerating the reaction even further.
How To Fix It:
When fibreglass is left exposed to direct sunlight, it will eventually yellow, fade, and lose its appealingly glossy finish. This happens because UV rays break down the resin matrix, leading to surface chalking, colour changes and a gradual weakening of the laminate. Over time, the once-smooth finish can become dull and brittle, making the structure more vulnerable to wear and weathering.
Yellowing isn’t just cosmetic - it’s a sign that the surface protection is failing. Without treatment, UV damage can penetrate deeper into the laminate, reducing strength and shortening the lifespan of your fibreglass.
How To Fix It:
When fibreglass is cut, the exposed edges can often fray or splinter, leaving behind sharp strands and loose fibres. Even light sanding may pull these fibres free, creating weak spots and an untidy finish. Not only does this look unsightly, but if left untreated, these rough sections can also wick in moisture, leading to further damage and potential delamination over time.
Rough edges and lifting fibres aren’t just a cosmetic nuisance - they compromise both the strength and safety of the laminate.
How To Fix It:
Yes - proper safety gear is essential when working with fibreglass. The process creates fine dust particles that can irritate your skin, eyes and lungs. Resins, catalysts and solvents release fumes that may be harmful if inhaled or absorbed through contact. Without the right protection, you could face issues like skin rashes, eye irritation, breathing difficulties, or even long-term respiratory problems.
Safety Steps: