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Gelcoat and Polyester Resin FAQs

What is gelcoat?

Gelcoat is a polyester coating that typically serves as the finish coat for molded fiberglass construction projects. Gelcoat also lends itself well to repairs of existing gelcoated parts that have been damaged or otherwise compromised.

Boat builders frequently use gelcoat as the final step in the fabrication of a new fiberglass boat hull. It protects the layers of fiberglass from water intrusion and subsequent damage. After applying the last layer of fiberglass, a skilled marine fiberglass worker applies gelcoat to create a smooth gloss finish. Although relatively durable, it accumulates scratches and dings that require cosmetic repairs.

What is gelcoat used for?

Gelcoat is best known for its use as a finish on fiberglass boat hulls. While it protects the underlying fiberglass layers from water intrusion, with older boats sometimes water permeates into the fiberglass under certain conditions. This leads to osmotic blisters that can weaken the fiberglass and are labor intensive to repair.

In addition to putting the finishing touch on fiberglass hulls, many trailered racing sailboats feature bare gelcoat bottoms. Most RV exteriors are also made of fiberglass, with gelcoat finish applied over the fiberglass shell.

Two varieties of gelcoat exist, each with specific properties and purposes. Soft general-purpose gelcoat is typically applied to more dynamic surfaces, such as boats. Because this gelcoat is flexible, it adjusts to a boats movement, so the chances of cracking are minimized.

Tooling gelcoat is harder and less flexible, and is often used on stable surfaces such as bathtubs and countertops.

What is the difference between gelcoat with and without wax?

Wax-free gelcoat is an important component of a multi-layer wet lay-up application. While building up your layers, you do not add wax to polyester resin. Added wax will not allow the strongest chemical bond between the layers.

How does gelcoat cure?

Typically made of liquid paraffin and styrene, wax additive is used in the final layer in order to properly cure. The wax forms an air barrier, enabling the resin to cure to a tack-free state that is ready for sanding and finishing work.

How do you fix sticky fiberglass gelcoat?

When gelcoat does not cure correctly, there is a handful of major potential culprits. Too little MEKP catalyst, forgetting to use a wax additive in the final coat, not applying gelcoat in the wet film thickness needed to cause the exothermic reaction, and applying more gelcoat over a layer that has not cured. Your best bet in this situation is to scrape it off and start over.

What causes cracked, crazed gelcoat?

Fiberglass boats of all vintages are prone to stress cracks in the gelcoats surface. The stress cracks first show up as fine hairline cracks, but often expand over time. This expansion can potentially cause the underlying structure to delaminate, often leading to resin swelling and osmotic blistering.

Stress cracks generally form in locations in which the fiberglass is under higher-than-usual stress. Examples include attachment points for anchor pulpits and rail stanchions, along with other rigging-related stress areas. If the boat was involved in a rub-rail collision, or someone dropped a heavy object in a specific spot, stress cracks are a common result of those incidents.

Maybe the crack resulted from an inadequately supported equipment installation, such as a poorly mounted winch. In this case, the user must address the underlying problem before performing cosmetic repairs.

Gelcoat expansion and contraction can also cause stress cracks over time. Consistent wave action against the hull is another potential culprit. Finally, direct sunlight on dark-colored gelcoat can gradually cause delamination and blistering.

Gelcoat crazing produces similar fine cracks, but they come from a different source. Older fiberglass boats typically featured overly thick fiberglass layup, which gradually led to development of the tiny cracks on varied parts of the boat.

Most stress cracks are limited to the gelcoat, and you can easily repair them with the right supplies and some elbow grease. If you ignore the cracks, water can gradually seep into the fiberglass and lead to delamination and blistering. If you suspect structural damage, it is time to get a professional inspection before you attempt any repairs.

How do you apply gelcoat on fiberglass? Can you apply gelcoat over gelcoat?

Applying gelcoat over an existing (fiberglass) gelcoat finish is a relatively straightforward process. Begin by sanding the surface to the underlying gelcoat so the new gelcoat can form a mechanical bond. Next, thoroughly clean the surface with a solvent like acetone.

When the surface is clean, prepare to begin the gelcoat application before dirt particles or dust start to accumulate again. Decide how many coats of gelcoat you will need. Note that if you plan to change colors, you will require at least two coats so the underlying color does not show through.

Apply the gelcoat with a roller, brush, or spraying system, 18 to 25 mils thick for proper curing. A mil gauge will help you to view the gelcoats thickness. Remember to use an MEKP catalyst. Next, apply a second coat followed with MEKP catalyst. The final coat needs wax that enables the gelcoat to dry without becoming tacky.

After the gelcoat cures, sand and buff it, working with a 4x4 area and keeping the buffing pad clean. Afterward, apply a machine glaze and two coats of wax for an impressive result.

Important note: Before beginning your gelcoat application, assemble all your supplies and decide on the application sequence. After you mix the gelcoat, you will only have about 15 minutes to apply the material before it begins to harden.

Your actual working time depends on the quantity of catalyst and the ambient temperature. If the air temperature is below 60 degrees F, the gelcoat will not properly cure. As the temperature rises, your working time quickly decreases.

How do you tint or color match gelcoat?

Dispersion pigments or colorants are used with polyester resins, epoxies, and white or neutral gelcoat to create custom colors and match existing finishes when used for repairs. Note that lighter colors require roughly 6 to 8 fl oz of pigment per gallon of resin and 4 fl oz per gallon for darker colors. Refer to color match charts to determine the best color gelcoat to start with. For example, add black or brown pigment to white gelcoat to create off-white.

How long does gelcoat take to cure or dry?

If the user has added the proper amount or ratio of catalyst, the gelcoat will typically cure overnight. If the temperature is 70 degrees F or above, allow about four to eight hours of curing time.

How thick is a gelcoat finish?

Typically, gelcoat is applied in 4 to 5 layers at 15-20 mil thickness wet per coat. Thicker coats may be used below the waterline for blister prevention, but the thicker the gelcoat layer, the more prone it may be to stress cracks.

What is the difference between gelcoat and paint?

Gel coat is a colored resin that is buffed and polished to a durable gloss finish. Most marine paints are urethanes (monourethane, polyurethane). Gel coat requires more annual maintenance in the form of buffing, polishing, and wax. However, it is easy to touch up and buff out scratches.

Some builders (wooden boats, in particular) and DIY boaters prefer paints to refresh older boats. Most marine paints are easier to use, and they dry to a glossy finish without buffing and polishing. However, painting over gelcoat requires advance surface preparation before you can apply. Marine paint is less chemically stable, so it is not as durable as gelcoat.

Can you paint over gelcoat?

The short answer is yes, you can paint over good-condition gelcoat. If the existing finish does not have any major crazing or cracking, clean and prep the surface for painting.

Next, apply a coat of epoxy primer. Sand the primer, and then remove all sanding residue. Finally, apply the topcoat according to the manufacturers instructions.

Do you need to prime gelcoat to paint over it?

No. Most topside paints can be applied over gelcoat finishes that are not damaged, cracking, or flaking.

What is polyester resin?

When people talk about polyester resin, they are sometimes referencing an inexpensive, generic boatyard resin or fiberglass resin product that contains a mix of orthophthalic, isophthalic & low profile resins. Such all-purpose laminating resins works well for small repairs on soft wood, metal, and fiberglass. It usually contains wax, but not always. Do not use below the waterline.

But, polyester resin can also refer to the laminating and finishing resins used in layups, surfboard building and repair.

What is the difference between polyester laminating resin and finishing resin?

Polyester laminating resin is primarily used for saturating fiberglass fabrics and keeping them tacky while you are building up the layers. Because the surfaces remain tacky, the fiberglass cloth or mat layers adhere to each other very well.

Laminating resin does not contain any wax, and it will not cure by itself when it contacts air. Polyester finishing resin is used as the final coat for building and repair projects. You can apply this finishing resin over fiberglass, metal, and wood.

Polyester finishing resin does contain wax, so it cures properly when it comes into contact with air. When this resin cures, you will have a hard, non-tacky finish that is ready for gelcoat, sanding, or painting applications.

What is the difference between epoxy and polyester resin?

Both epoxy and polyester resins are frequently used in home, vehicle, boat-building, and aircraft repair applications. These versatile materials are also increasingly popular with crafters. With that said, however, the materials have seven notable differences.

Epoxy has a mildly objectionable odor, while the polyester resin smell is so strong it makes your eyes water. Always use proper ventilation when applying either substance in an enclosed area.

Polyester resin generally cures faster. Epoxy is significantly stronger than polyester resin, from a lbs per sq inch standpoint. Epoxy resin also exhibits better moisture resistance properties than polyester resin, which is actually permeable. Epoxy is more durable and chemical resistant. Polyester resin is a lot more economical, which is why it is so popular for low-stress or non structural applications

Can you paint over polyester resin?

Yes. Cured finishing resin may be painted or gelcoated over, it can also be wet sanded and buffed to a glossy finish.