What is Gelcoat on BoatsOwners of any type and size of boat readily agree that they wear many hats while involved in recreational boating: navigator, helmsman, electrician, mechanic, chef, and chandler — to mention just a few. Eventually, owners will likely have to make fiberglass hull repairs, filling less obvious, but important roles in making those repairs — those of chemist, painter, and aesthetician. 

For the boat owner, knowing the basics of 21st century boatbuilding and materials, such as fiberglass and gelcoat, as well as the chemistry involved, will help when repairs are necessary and how to approach repairs in a safe, cost-effective and timely fashion — without a degree in chemistry. For the boat renter, knowing what to look for when considering a boat for rental, in terms of hull condition, can help avoid serious underway issues.  

What is Gelcoat 

Simply stated, gelcoat is the outermost structural layer on a fiberglass boat hull, designed to protect the underlying fiberglass layers. Gelcoat is a compound that produces a high-quality finish on fiberglass surfaces and incorporates a lot of chemistry, using epoxy, polyester resin, a catalyst, and other chemicals to achieve its strength and water-resistant properties. When cured, gelcoat combines with fiberglass to produce a smooth and durable surface that retards hull weakening from water intrusion and ultraviolet light.

Fiberglass, on the other hand, is the basic structural material that gives boat hulls their shape and strength. It is composed of a plastic matrix and polymers, reinforced by fine fibers of glass that combines lightweight and inherent strength to provide a water- and weather-resistant finish. 

Like any finish, gelcoat is prone to dings, gouges, scratches, and deterioration. It is usually pigmented to match the hull color and is available as both a resin and a thicker, putty form, called paste. Over time, spider cracks in gelcoat that radiate from joints or intersections in cabin, deck, or hull surfaces can appear, which may indicate weaknesses in the underlying fiberglass, often evidenced by popping or cracking sounds when underfoot. Another gelcoat problem is blistering, which generally indicates water intrusion under the gelcoat.    

Most repairs to gelcoat scratches, chips, cracks, and worn areas are straightforward and easy. When the gelcoat surrounding a scratch is in good condition, the best choice is gelcoat paste, which fills and finishes the area in one application — but not a single step. Sanding and polishing are critical to blending the area with the hull. 

The following tips are a simplified step-by-step repair process that will assist in developing a basic understanding of the steps involved in making gelcoat repairs. 

Gelcoat Repair Tips 

  • If gelcoat spider cracks or blisters are evident on a hull, consider having a marine surveyor or a qualified boat maintainer determine the extent of the problem and to learn the best approach to repairing, before beginning repairs.  
  • Gelcoat repair kits, available at marine supply dealers, are generally good for tackling small to medium repairs. The kits usually contain small amounts of gelcoat paste, hardener, and pigments, as well as mixing sticks and sealing film.
  • State-of-the-art gelcoat resins, polymers, hardeners, and catalysts can pose potentially serious health and environmental hazards. Protect hands, arms, eyes, and lungs with appropriate protective gear.
  • The working time for chemicals involved in gelcoat repairs, especially the catalyst, is very short. Thoroughly read directions before embarking on a repair and have all necessary materials and tools within reach.
  • Understand the step-by-step processes, including curing times, to make the repair and arrange materials and tools according to the steps to save time, effort, and material
  • Pay attention to the weather before undertaking a fix. Many gelcoat chemicals will not work as intended in hot or cold weather extremes. Rain and humidity can also affect the quality of the repair.  
  • A safe location for doing gelcoat repairs and work area ventilation are critical. If doing the work in a place other than a boatyard, ensure that the boat is away from vehicles, other boats, and children. Additionally, make sure the area is properly ventilated. 
  • The chemical makeup of gelcoat is such that it will not fully cure in air, so sealing the repair area is necessary. Covering with plastic film, a piece of a sealable bag, or wax paper works well.

Steps to Repair Gelcoat

What is Gelcoat on BoatsAs mentioned above, making repairs to small areas of gelcoat damage can generally be handled without too much effort. The following steps for making repairs are offered only as a guide; instructions and environmental and personal health/safety precautions provided with repairs kits should be followed to the letter. 

  • Wash the repair area and rinse thoroughly. Once the surface is clean and dry, mark off the repair area with masking or painter’s tape.
  • Gouge out small, narrow cracks in the repair area until wide enough to fill with gelcoat paste. Sand with 220-grit sandpaper, then clean with acetone to remove residue that could affect the bond between the damaged surface and the gelcoat.      
  • Match the existing gelcoat color by starting with a white/neutral paste and gradually add small amounts of pigment. Mix several test batches then add hardener and let cure. Once the acceptable match is found, mix a final batch using the same ratio.
  • Use a putty knife to push the paste into the area and overfill. Force out any air pockets.
  • Once the gelcoat has fully cured, sand the area, starting with 220-grit sandpaper and finishing with 400- or 600-grit. To complete the sanding process, use a wet sanding technique. Finally, apply a coat of high-quality marine polish.
  • A huge challenge involved in gelcoat repairs is matching the gelcoat color to the surrounding hull area, particularly if the hull paint is faded. Once the color is as close as possible, apply the “leave well enough alone” approach. 

Gelcoat repairs can be tricky, but understanding how the chemicals used in making gelcoat repairs interact with each other and the steps used in making the repairs will help to make the effort effective and safe. Knowing what to look for before setting sail, in terms of hull condition, can help avoid serious underway issues.