Repairing your Fibreglass Hull


If you own a fibreglass boat, you’ll know there’s potential risks of damage to the hull, be it from a rogue wave while trailering, grounding, knocking a jetty or even dropping a tool. The gelcoat will take the brunt of the impact but in more serious cases the underlying fibreglass can end up with some pretty nasty gouges. Ageing fibreglass can also be susceptible to cracking, crazing and blistering though with the right tools and a little know-how, you can get them patched up and have your boat looking great again.

Fibreglass Boat Materials

Most fibreglass boats are made using multiple layers of reinforced fabric and core materials held together with a polyester resin. The fibreglass is protected beneath a layer of gelcoat that’s around 12 to 22 mm thick and acts as a moisture barrier while also providing a smooth, glossy finish. Repair jobs needed on fibreglass boats are usually cosmetic, but if you think a crack goes through the laminate due to constant flexing for example, you may need to have a professional boat repairer take a look at it.

What you will Need for the Job

Products needed to complete a gelcoat repair include cleaner, gelcoat repair filler, hardener, painters masking tape, rag or newspaper, acetone, sandpaper, gloves, putty knife, a cup and stirrer for mixing, buffing compound and wax.

For fibreglass, you will need a fibreglass repair kit that usually consists of fibreglass matting, an MEKP hardener, brushes, polyester resin and a set of gloves. You will also need acetone and putty filler.

The majority of the links below are to Septone manufactured products and they also make a fibreglass repair kit. These products are also useful if you need to patch up surfboards, water skis, kayaks and plywood boats. 3M also manufactures quality gelcoat repair and protection products.


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Step by Step Instructions - Gelcoat Repair

  1. Remove any contaminants on the surface like wax, oil or grease with a hull cleaner.

  2. Clean any ragged edges of a gouge using a hand-held grinder. If you’re repairing cracks, use a sharp V-shaped tool to scrape to the bottom of the cracks and remove the damaged material. Remove any loose particles.

  3. Use a 100-grit sandpaper to remove any small bumps and ridges before cleaning with acetone.

  4. Tape off the damaged area using painters tape but leave a one centimetre margin. Cover with a rag or newspaper any parts of the boat not being repaired in case of product spills.

  5. Mix a batch of gelcoat repair filler then add hardener (included with the gelcoat repair filler) following the manufacturer’s instructions. Mix thoroughly so the batch cures at the same time.

  6. Use a putty knife to spread the gelcoat repair filler over the damaged area making sure you have enough product to cover the damage and to sand back. You should have 10 to 15 minutes before the gelcoat hardens.

  7. Once the product is completely hard, wet-sand the area using a 320-grit wet/dry paper with a sanding block to smooth the edges and flatten any bulges to make it even with the existing gelcoat. Finish off with a 1,000-grit paper.

  8. Buff out any imperfections with fibreglass buffing compound then finish with a wax.        

Step by Step Instructions - Fibreglass

  1. Prepare the surface with a wax and grease remover to ensure all dirt, grease oil and contaminants are removed. Let dry completely then cut away damaged area and any loose pieces.

  2. Sand the surface with 80-grit sandpaper to remove all primer, paint, rust or gelcoat. Sand 2-5cm beyond damaged area and down to bare metal or fibreglass. Repair both sides of the damaged area if possible for added strength. Depress or slightly bend the edges inward. Remove all dust created from sanding with a dry cloth or compressed air, then re-clean the surface with acetone.

  3. Using the Fibreglass Repair Kit, cut the fibreglass matt to the required size and allow an extra centimeter around the edges. On tight curves tear the mat to fit, adding extra pieces if needed. Put the mat on a clean surface such as newspaper until it’s ready for use.

  4. Mix no more resin than what you can use in 30 minutes, 250ml of resin will cover approximately 0.25m² of fibreglass matting.

  5. Mix the resin thoroughly with the Septone MEKP catalyst (included in the Fibreglass Repair Kit): For 50ml resin on a cold day, use 40 drops of catalyst. For 50ml of resin on a hot day, use 20 drops of catalyst.

  6. Brush a coat of mixed resin on the damaged area extending 5-10cm beyond the repair. Place the first layer of fibreglass matt onto the mixed resin then in a dabbing motion use the paintbrush to saturate the matt and remove any air pockets. Continue applying additional layers of matting and mixed resin as needed while the first application is still sticky and wet. If the surface dries hard and is no longer sticky, sand with 80-grit sandpaper before applying additional layers of mixed resin and matting. Follow the same procedure if both sides are being repaired.

  7. Allow area to cure, approximately two hours at around 23-34°C. Curing will take longer at cold temperatures. Once cured, sand repaired area with dry 80-grit sandpaper. Clean area with acetone.

  8. To return the contour or to smooth out low areas in the surface, use a layer of Eazefill Putty with hardener thoroughly and apply a thin layer of filler to the repair surface using firm pressure on the applicator to force the filler into all crevices and eliminate any air pockets. Gradually apply the remaining putty until it’s slightly higher than the edge of the repair.

  9. Allow 20 to 30 minutes for the putty to cure. Sand with 180-grit sandpaper. Ensure you feather the edges then re-clean with acetone.

  10. Mask off the repair area in preparation of the primer coat. Apply Eazefill Putty over the repair area in thin layers, three-to-four coats. Leave five minutes dry time between each coat. After the final coat leave to dry for four hours or overnight.

  11. Remove all masking tape/paper and start sanding repaired area with wet and dry 1200-grit sandpaper. Feather edges and sand until smooth.

If you need some advice on the best products for repairing the fibreglass on your boat, call Boat Accessories Australia on 1300 308 161 or send us an email.  

Category: Maintenance

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