Fiberglass Repairs Made Easy: Common Mistakes and How to Avoid Them

Whether you’re a boat owner, a DIY enthusiast, or someone who enjoys working with fiberglass, repairing it can feel intimidating at first. From cracked surfaces on your boat to damaged panels on a car or even a household project, fiberglass repairs require precision and patience. When done correctly, they can restore not only the function but also the appearance of your fiberglass structures. But when done improperly? Those errors can snowball into larger, more expensive headaches.

This guide will walk you through the most common mistakes people make when repairing fiberglass and, most importantly, how to avoid them. By the end, you’ll feel confident tackling your next repair project, saving time, money, and effort.

Why Fiberglass Repairs Can Be Tricky

Fiberglass and resin, while strong and versatile, can be sensitive to incorrect handling. Minor errors in preparation or application can compromise the durability of your repair. Failing to address issues at the start can result in wasted materials, uneven finishes, or repairs that don’t last.

The good news? By understanding the top mistakes and how to steer clear of them, you’ll set yourself up for success.

Mistake 1: Skipping Proper Surface Preparation

One of the most common mistakes in fiberglass repair is failing to prep your surface adequately. Whether it’s dirt, grease, or lingering damaged materials, these contaminants can prevent your repair from properly bonding to the surface.

How to avoid it:

1. Clean the area thoroughly using acetone or a similar solvent to remove grease, oils, and dirt.

1. Sand the damaged area well. Fiberglass repairs rely on mechanical bonding, and a smooth, glossy surface won’t allow for proper adhesion. Use 80-grit sandpaper to roughen the area and create a good bonding surface.

1. Remove loose or weak fibers from the damaged area. If a crack or tear exposes deteriorated fiberglass fibers, trim, dremel and/or sand them away before proceeding.

Taking the time to properly prep your surface is your first step toward a long-lasting fiberglass repair.

Mistake 2: Using the Wrong Resin

Fiberglass repairs typically rely on polyester, epoxy, or vinyl ester resins. Each resin type has different properties, and using the wrong one can result in weak repairs or compatibility issues with existing fiberglass.

How to avoid it:

• Understand fiberglass compatibility. Most fiberglass reinforcements are compatible with polyester, vinyl ester and epoxy resins. The exception to this, is chopped strand mat. Chopped strand mat is not compatible with epoxy resins. Mat needs a polyester or vinyl ester to breakdown the styrene binder that holds the fiberglass fibers together.

• Understand resin compatibility. Polyester resin won’t bond well to epoxy, but epoxy can bond over top of polyester. Choose your resin carefully based on the material you’re repairing. If you are going to use a gel coat over top, make sure you use a polyester or vinyl ester resin for the repair.

• Follow manufacturer instructions carefully. Skipping steps or altering the mix ratios of resin and hardener can impact its curing process and lead to weak repairs.

By choosing the right resin and mixing it the way the manufacturer recommends, you can avoid issues before they even arise.

Mistake 3: Rushing the Curing Process

It's tempting to want rapid results, especially if you're pressed for time—but rushing the curing process is a recipe for disaster. A poorly cured repair not only weakens the bond but can also lead to warping, uneven surfaces, uncured spots or premature damage.

How to avoid it:

• Mind the temperature. Fiberglass resins cure best in a controlled environment, typically between 70°F to 80°F. Avoid working in cold or damp conditions, as these can interfere with curing.

• Be patient. Allow the resin to cure fully before sanding, painting, or putting the repaired fiberglass to use. Rushing risks surface imperfections or structural weaknesses.

• Avoid shortcuts like heat guns unless used properly. They might seem like they’ll speed up curing, but they often result in uneven reactions within the resin.

Investing time in curing ensures your repair will have maximum strength and durability.

Mistake 4: Overlooking Safety Precautions

Working with fiberglass involves various chemicals and fine particles that can harm your health if not handled properly. Many DIYers overlook safety gear, leading to potential skin irritation, harmful fumes, or eye injuries.

How to avoid it:

• Wear protective gear. Use gloves, safety goggles, and a respirator to shield yourself from chemical and particle exposure.
• Work in a ventilated area. Fiberglass resin emits strong fumes that can irritate your lungs. Ensure adequate airflow in your workspace.
• Manage dust carefully. Sanding fiberglass creates fine, sharp particles that can irritate the skin or cause respiratory issues.

Making safety a priority ensures that your repair is not only effective but also injury-free.

Mistake 5: Not Layering Fiberglass Correctly

Fiberglass repairs often involves layer upon layer of material to create a strong, reinforced patch. Improper layering—too few layers, incorrect sizes, or uneven application—can weaken the repair, resulting in cracks or failure under stress.

How to avoid it:

• Cut fiberglass cloth into progressively larger layers. Start with a small patch to cover the immediate damage, and use larger patches with each additional layer to create a tapered, seamless repair.

• Apply resin evenly between layers. Each layer of fiberglass cloth needs to be fully saturated with resin; avoid dry spots! Use a roller or brush to smooth each layer.

• Don’t skimp on layers. Even if the repair looks fine with two or three layers, additional layers may add much-needed strength. 

Proper layering habits ensure your repairs hold up under pressure—literally.

Mistake 6: Neglecting the Final Finish

Once the structural repair is done, it’s easy to overlook the finishing touches. But a poorly finished repair, with visible patches or uneven surfaces, can detract from your craftsmanship and even weaken the structure.

How to avoid it:

• Sand with precision. Use fine-grit sandpaper (120 to 220 grit) to smooth the surface without over-sanding and damaging layers.
• Apply gel coat. A gel coat restores the original look of boats and other fiberglass surfaces, as well as protects the material beneath from UV rays and wear. (Do not use over epoxy resin)
• Polish for a professional look. Using waxed resin/gelcoat or adding a wax ensures a tack-free, polished finish.

Finishing your repair well ensures it not only looks great but also lasts for years to come.

The Bottom Line? Quality Over Speed

When it comes to fiberglass repairs, preparation and patience make all the difference. Each step is an opportunity to avoid costly mistakes and ensure your repair is durable and professional-looking. Remember to:

• Prep your surface properly.

• Use the right resin for the job.

• Allow adequate curing time.

• Work safely and mind the details of layering and finishing.

If you’re unsure where to start or need extra help, consider reaching out to us at support@truecomposites.com for guidance on repairs or products.