Banksphotos / Getty Images

How to Fix Chips, Cracks, and Holes in Fiberglass

Originally posted on Men's Journal | By Max Winters | Last updated Dec 4, 2024 7:00 AM ET

Fiberglass is a versatile and durable material... that's why it's used in everything from boats to car panels to household items. But what happens when the cosmetic finish chips, or the original laminate cracks? Over time (or after an incident) this can happen. The good news is, the art of fiberglass repair is within reach, and we know you can do it. Go ahead, grab your safety goggles, choose the right fiberglass epoxy resin, and we know you'll be able to do the entire repair yourself.

Is it fixable? Things to Consider

Types of Repairs: There are levels of repair options. Small cracks, chips, and holes are typically manageable cosmetic damage. Large structural failures might require more demanding applications and therefore, professional help.
Location: If the damage requires joint repair, or affects a load-bearing area or safety-critical part, consult an expert.
Purpose: For cosmetic repairs, a simple patch-up might suffice. For functionality, you’ll need to ensure strength and integrity.
Material Match: Fiberglass comes in different grades and resin types. Matching the repair material to the correct epoxy filler is essential for a stronger finish.

Steps to Repairing Fiberglass

Materials and Tools You'll Need

Fiberglass cloth or mat
Resin (polyester, epoxy, or vinyl ester, depending on your project)
Hardener or catalyst (for the resin)
Sandpaper (various grits, from coarse to fine)
Orbital sander (optional based on size of repair patch)
Utility knife or scissors
Mixing container and stir stick
Protective equipment (gloves, safety glasses, dust mask)
Acetone solvent (for cleaning surfaces)
Paintbrush or roller (for resin application)
Plastic squeegee
Vacuum (for glass fibers)

1. Set Yourself Up

Wait for the Correct Conditions: Wait for clear weather when you have adequate time.
Prepare the Area: Gather all of your supplies in a well-ventilated area.
Safety First: Because the glass fibers can pierce your skin and exposure to dust can aggravate your lungs/eyes, don't skip the safety goggles, gloves, and dust mask.

2. Prep the Area

Clean the damaged surface: Wipe the surface with acetone to remove dirt, grease, or residue to get a clean surface for the fiberglass resin adhesion.
Remove weak or loose fibers: Use a utility knife to cut away any broken pieces or frayed fiberglass. Wear gloves to protect against hairlike glass fibers.
Sand the edges: Feather out the edges of the damage to create a smooth transition zone for the repair patch.
Tidy up: Clean the repair surface of excess dust because it can affect the entire bonding process.

3. Measure and Cut the Fiberglass Cloth

Cut to size: Trim the chopped strand mat (CSM) so that the piece of fiberglass cloth slightly larger than the damaged area. If needed, prepare multiple layers of this core material.
Layer planning: Plan for smaller inner layers and a larger outer layer of glass fibre for added strength and a smooth finish.

4. Mix and Apply the Resin

Follow instructions: Mix the correct ratio of resin and hardener according to the manufacturer’s directions.
Brush on the resin: Apply a thin layer of mixed resin over the damaged area as a base coat. Extra resin won't increase the degree of reinforcement, stick to lightly saturating the layer of glass fibre matting.
Position the cloth: Lay the largest piece of fiberglass cloth on top of the resin. Use a paddle roller or plastic squeegee to press it down and remove air pockets and smooth excess epoxy.
Layer up: Repeat with additional layers of cloth and resin. There should be some excess resin built up and slightly higher than the surrounding area.

5. Cure and Sand

Let it cure: Allow the repair to harden completely—drying times could take several hours to a full day depending on the resin type.
Sand it smooth: Start with coarse-grit sandpaper to shape the repair, then switch to finer grits for a smooth finish where the patch meets the primary structure. The same applies to machine-based sanding.
Clean up: Vacuum up the fiberglass dust as you work.

6. Finish with Gelcoat or Paint (Optional)

Apply an outer layer: Apply a thin layer of topcoat resin, pigmented resin, or gelcoat finish to seal and protect the level surface.
Paint and polish: For painted surfaces, prime and paint the repair to match the original color.

Why would a fiberglass repair fail?

Insufficient Preparation: Correct surface preparation is vital so skipping cleaning or sanding is going to affect the bonding surface. Using the incorrect tools for the repair process can cause poor adhesion and keep you from a perfect finish.
Incorrect Resin Mix: Incorrect mixing by improvising the amount of hardener with incorrect ratios affects the bond strength.
Air Bubbles: Trapped air pockets will weaken the patch and can lead to cracking. This can be
Dirt and Debris: The epoxy's exposure to dust, dirt, or glass particles, will disrupt the physical bonding of resin to the fiberglass structure.
Inadequate Layering: Thin or uneven layers of your fiberglass patch compromises structure and causes a lack of tensile strength.
Improper Curing: Respect the resin cure time. It can take longer during colder conditions. Rushing the drying time can compromise durability of a structural repair.
More Severe Problem: Environmental conditions (rain, cold, humidity) or more damage to the base structure can create problems that need an alternative method or extra reinforcement.