Boat Repair: How to repair cracks and scratches in 8 easy steps | Norton Abrasives

Boat Repair: How to repair cracks and scratches in 8 easy steps

speedboat in the sea

No matter how careful you are, at some stage in your boat’s life cracks and holes are likely to appear. Without immediate and ongoing maintenance & repair, these small damages could escalate into a much bigger problem.

This article looks at how to repair a deep scratch or crack in the hull

Boat Damage

Boat owners will recognise that cracking, crazing, and blistering in gelcoat are commonplace. These blemishes make up the majority of the ongoing repair and maintenance work that an owner must undertake; whether privately or professionally.

Most cracks or crazing appear gradually over time and are the result of the outer surface of the boat flexing. Prime areas for cracking are locations where laminate is found, typical examples are the curved deck to cabin panel, around/near windows, and/or the bulkhead.

It follows that the longer cracks are left, the more extensively and deeply they will spread; until eventually the integrity of the panel will become compromised.

Identifying and appraising the damage

Permanent repairs to this kind of damage are normally undertaken by using Epoxy resin fillers or fibreglass/gelcoat fillers. Epoxy resin kits are numerous and readily available to purchase online.

Colour matching may present a challenge when using off-the-shelf epoxy resins but when properly applied, epoxy resin provides an extremely durable, water-resistant seal that can be properly finished and painted over. Fibreglass and gelcoat mixtures might present a better option for colour matching.

Firstly, assess the damage: locate the areas that require attention and determine to what extent. Decide if the crack was caused by impact or by flexing; this will for the most part, be obvious.

Flexing cracks appear as multiple, long, thin, flat lines. Impact cracks can be more rounded or circular in appearance and usually an indentation is apparent.

Repairing cracks, blisters and deep scratches


Grind a shallow bevel around the damaged area, to cut back the crack or open the blister, this can be done with a Die Grinder and a Norton SpeedLok™ disc or carbide burr.

You may need to sand down any sharp edges or burrs with sandpaper or a sanding sponge; starting with 80 to 150 grit and ending with 240 grit. Wipe the area clean to remove any dust or debris.


Now, prepare the filler. If you are using a store-bought epoxy resin filler, stick to the guidance issued by the manufacturer in its preparation. If you opt for a fibreglass/gelcoat mix, you’ll need powdered fibreglass and a coloured gelcoat to make the filler mixture.

Your boat’s exact colour will be available through the manufacturer, take note of its serial number, make and model to ensure you get the right one.

When mixing the filler, ensure that you stick to the manufacturer’s guidelines with regards to ratios and consistency. As a guide, think peanut butter consistency; it should be spreadable but not too thin.


Apply the filler over the damaged area with the flat end of a mixing stick or a plastic spreader. Gradually fill the scratch or scratches, ensuring that the filler is pressed into the fibres and shape it to match the surface contour using a plastic spreader.

Leave the mixture slightly higher than the surrounding area and remove any excess before it begins to cure. Allow the epoxy to cure thoroughly.

The length of the time to cure will depend on a number of factors; temperature, humidity and consistency of the mix. Touch test the cured filler, if it’s not cured, the area will feel sticky.


When you’re satisfied that the filler is fully cured, sand the area to blend it in with the surrounding contour.

Use Norton Black Ice 50 grit sandpaper or a sanding sponge wet or dry to remove any obvious bumps or ridges, then move up to 80 grit to further smooth the surface, and finish with 240 grit when you are close to the final shape.

For larger filled areas, use a DA or orbital sander with Norton Multi-Air discs or strips, which offer great dust extraction, for a smoother finish and less clean-up time.


If there are any remaining voids or surface imperfections, repeat steps 3 and 4.


Next to re-spray the filled and sanded area. To do this, mix gelcoat, gelcoat reducer and hardener together in the ratios indicated by the manufacturer’s instructions.

A disposable spray can be the best way to evenly apply the mixture across the surface. Spraying gelcoat is a lot different to spraying paint, it can be hard work to use, quite messy, and will not provide a smooth looking finish in the first instance.

Further process steps are required in order to get your glossy, continuous finish. Wait for the gelcoat to cure and harden before sanding.


Sand the repaired area gently with Norton Black Ice 800 grit wet and dry sandpaper, then finish with 1200 grit.


Now polish the area using a polishing compound and a polishing pad. We recommend Norton Ice Xtra Cut with Norwool lambswool or white hard foam polishing head.

Be sure to apply the polishing compound directly to the pad and work on small areas of the surface using light pressure and low speed.

Wipe with Norton Blue Magnet microfiber cloth as you go along.

For more information

Achieve this repair with the help of Norton Abrasives. We have a great range of abrasives and non-abrasives that are perfect for marine applications. For more information, download our new marine brochure or contact us using our web form.