The Ultimate Guide To Replacing Gelcoat On Your Boat

Gelcoat is a special resin that is sprayed on the outside of a fibreglass boat to give it its colour and shine. It also acts as a water barrier to the fibreglass underneath. Over time, gelcoat can become dull and chalky, and may need to be replaced. This is a labour-intensive process and involves several steps, including cleaning, degreasing, and compounding the gelcoat. The process of re-gelcoating is different from painting, although both require the same amount of prep and fairing. Gelcoat is usually sprayed on in multiple coats to achieve the desired thickness.

Repairing gelcoat on a fibreglass boat

Gelcoat repairs are inevitable for most boats, especially racing boats, after a sustained period on the sea. Repairs are often needed for spider web cracks or eroded gelcoat, where the base laminate is starting to show. The good news is that most gelcoat repairs are easy to undertake and will save you lots of money by doing them yourself.

Prepping the Damaged Area for Repair

First, chip or grind down any damaged material and finish feathering the edges with coarse sandpaper (240-grit). This will give the new layer of gelcoat a suitable area to bond with. Be careful not to sand down too far and reach the glass. Once the area has been sanded down, wipe it with acetone to remove any contamination.

Colour Matching

Before you can repair the gelcoat, you'll need to colour match it to the rest of the boat. FibreGlassDirect supplies six standard gelcoat colours: white, black, dove grey, dark olive, bright blue, and clear. The clear gelcoat can also be pigmented if required. One or more mixes of pigment may be required to achieve an exact match.

Mixing the Gelcoat

Mix the gelcoat with 2% catalyst based on an average room temperature. This will give a working time of approximately 20 minutes. In colder conditions, you can add more catalyst, but generally no more than 3%. In warmer conditions, less catalyst may be needed to slow down the curing time, but no less than 1.5% should be used. If the gelcoat is allowed to cure too quickly, it will create air bubbles that will appear when sanding takes place after the cure.

Applying the Gelcoat

Depending on the depth of the repair, apply the first layer with a brush (for very shallow repairs, a matchstick may be used) so that it is thick enough to cover the damaged area and overlapping the masking tape. Remove the masking tape before the gelcoat fully cures. This will leave a clean edge without disturbing the hardened new layer of gelcoat. After applying the gelcoat to the repair, cover the repair with sandpaper, ensuring that there is no trapped air. This will help reduce sanding when the repair is dried. If two layers are required, it is important to sand the first layer once cured to allow the new coat to stick.

Sanding and Polishing

When the gelcoat has fully cured, the area should be sanded. Start with a coarse sandpaper such as 240-grit and work up to 1000-grit. Sand the area so that it becomes flush with the untouched area and blend the edges accordingly, finishing with the 1000-grit sandpaper. To add shine and to remove any of the finer scratches from sanding, a handheld, motorised polisher may be used with a lamb's wool pad. Farecla compounding pastes work particularly well for adding shine.

Preparing the gelcoat mix

Before preparing the gelcoat mix, it is important to determine the batch size required for the repair. A rule of thumb is that 1 gallon of gelcoat is needed for approximately 80 square feet of the boat's surface. For smaller repairs, you can estimate that 1 quart of gelcoat is needed for 20 square feet, or 1 litre for 2 square meters.

Once you have determined the batch size, it is time to tint the gelcoat to match the colour of your boat. If your boat is new and the manufacturer is still in business, you may be able to get gelcoat that closely matches the original colour. If not, you will need to purchase gelcoat resin and tint it yourself. Select a base gelcoat colour that is closest to your boat's existing colour, and then adjust it with pigments to get a good match. This step may require some trial and error, as matching the repair colour to the boat is one of the hardest parts of the job. It is best to work in bright daylight or use a sun gun to simulate true daylight conditions. Apply small amounts of the uncatalyzed gelcoat to the repair area and wait for the solvents to flash off, which will allow you to see any colour variations. Adjust the colour by adding small amounts of tinting pigments, thinking in terms of the basic colours (e.g. black, red, blue, green). You may need to dilute the pigments with white gelcoat to weaken their effect. Remember to wipe away all test smears with acetone or lacquer thinner before proceeding to the next step.

After achieving the desired colour, it is time to add the catalyst to the gelcoat mix. The amount of catalyst will depend on the temperature and your preferred drying time. A higher percentage of catalyst will result in faster drying. For example, at a temperature range of 79-85°F, use 4-6 cc of catalyst for a quart of gelcoat, 23-26 cc for a gallon, and 110-130 cc for a 5-gallon batch. Be sure to follow the gelcoat supplier's recommendations for catalyzation to ensure proper curing.

Finally, mix the gelcoat to your desired consistency, thinning it with a styrene thinner if spraying. Be careful not to over-thin the gelcoat, as this can affect the quality of the finish. Now you are ready to apply the gelcoat to the repair area!

Adding catalyst and applying gelcoat

The amount of catalyst (MEKp) required depends on the gelcoat manufacturer. Most gelcoat manufacturers list the amount of catalyst required on the side of the can. The ideal range is 1.8% @ 77°F (approximately 12 drops per ounce of gelcoat). If the gelcoat does not get enough catalyst, it will not harden. If the measurement is off even slightly, the gelcoat could start to harden but not cure completely, leaving a tacky, non-sandable surface.

It is also possible to add too much catalyst to the mixture, which will cause the gelcoat to start curing in the can or while you are applying it. This is more likely to happen when mixing larger batches of gelcoat. Always mix in small batches. Generally, mix one-pint batches.

The higher the percentage of catalyst, the faster the gelcoat will dry. The colder the air temperature, the higher the percentage of catalyst you should use. Do not go below 1/2% or above 2%. For warmer weather, use less MEKp, and for cooler weather, use more.

Divide the batch into a 2/3 portion and a 1/3 portion. Mix the 2/3 portion of matched gelcoat to the desired consistency. If spraying, a styrene thinner is best to dilute the viscosity for spray application. Mix in the catalyst following the recommendations of the gelcoat supplier. Over or under catalyzation may prevent the product from reaching a proper cure.

Apply this mixed gelcoat to the surface of the repair with a spray gun or foam brush. Apply several light coats, feathering each one farther from the repair area. Allow the solvent to flash off between coats. You may need to apply five or six coats to hide the shadow of the repair. Most gelcoat colours will dry lighter in colour if they are too thin. Most manufacturers recommend a total film thickness of 15 to 20 mils.

Make another mixture with the 1/3 portion of gelcoat and thin to the desired consistency, and again, add the catalyst. Spray the first coat of this mixture over the previously sprayed area. Then apply two or three more coats of this mixture over the entire area. Extend each coat farther than the previous one, with the last coat out to the tape line.

Tips

• If using a wood mixing stick, place the stick in the resin before adding the catalyst so the wood doesn't absorb the catalyst.
• Only catalyse slightly more than needed. Resin that cures still in the mixing pot is unusable.
• Gelcoat can also be rolled or applied by a foam brush in patch areas, but spraying tends to yield the best results with the least amount of follow-up sanding.

Sanding and polishing

Start by sanding the gelcoat with 180-grit sandpaper to remove any imperfections and create a smooth base. It is important to work in a well-ventilated area as the process will generate dust. Once you have achieved a satisfactory surface, progress to a finer grit sandpaper, such as 320-grit, to further refine the finish. Remember to sand in the direction of the gelcoat pattern to avoid damaging the surface.

After sanding, it is essential to remove any dust or debris from the surface. Use a tack cloth or a damp cloth to ensure the area is clean.

Now, it's time to begin polishing. Select a polish designed for use on gelcoat or fiberglass and apply it to a small area using a soft cloth. Work the polish in a circular motion until the surface becomes glassy. Take your time and be careful not to overheat the surface, as this can cause staining.

Once you have polished the entire surface, it is important to protect your work with a coat of wax. Apply the wax with a cloth or foam pad, following the instructions on the product you have chosen. Allow the wax to dry and then buff it to a shine using a soft, clean cloth.

Finally, stand back and admire your handiwork! Your boat's gelcoat should now have a restored shine and a smooth finish. Regular maintenance, including waxing and polishing, will help to prolong the life of the gelcoat and maintain its appearance.

Painting vs re-gelcoating

When it comes to boat maintenance, there are two main options to consider: painting or re-gelcoating. Both methods have their own advantages and disadvantages, and the best choice depends on various factors such as cost, time, durability, and aesthetics.

Re-gelcoating is the process of applying a resin-based finishing material to the hull of a boat. It serves as a protective layer, shielding the boat from UV rays and harmful chemical reactions. A well-maintained gelcoat can last up to 15 years or even two decades, requiring minimal repairs during this period. However, the application process is labour-intensive and requires a high level of skill and patience. It is also relatively expensive, especially if you are re-gelcoating the entire hull from scratch.

On the other hand, painting is a more practical and affordable option. Marine paint offers protection against UV rays and chemical reactions, but to a lesser extent than gelcoating. Painted hulls are resistant to cracking and peeling, but they are more vulnerable to chipping due to their lower chemical stability. Painting is generally easier and less time-consuming than gelcoating, making it a popular choice for DIY repairs. It also offers a wider range of colour options to choose from. However, the major downside is that repainting is typically required every year or two to maintain the boat's longevity.

When deciding between painting and re-gelcoating, consider factors such as cost, time, durability, and the condition of your boat. If you have the budget and skills for re-gelcoating, it can be a more cost-effective long-term solution. On the other hand, if you are short on time or prefer a wider range of colour options, painting may be the better choice. Additionally, if your boat's gelcoat is still in good condition, you can opt to paint over it, adding an extra layer of protection and enhancing its shine.

Ultimately, both methods have their advantages, and the decision should be based on your specific needs and circumstances. Regular maintenance and care, regardless of the chosen method, will ensure the longevity and optimal performance of your boat.

Frequently asked questions