Gel coat is the colored, protective outer layer of fiberglass boats. It's a specialized polyester resin that forms the visible surface of the hull, deck, and other fiberglass components. Unlike paint, gel coat is integral to the boat's construction. It's applied to the mold first, then fiberglass layers are built up behind it.
If you own a fiberglass boat, understanding gel coat matters. It's what gives your boat its color, gloss, and first line of defense against the elements. And when it starts to fail, the problems go deeper than appearance.
How is gel coat made and applied to fiberglass boats?
Gel coat is made from polyester resin mixed with color pigments, UV stabilizers, and curing catalysts. During boat manufacturing, it is sprayed directly into a polished mold before fiberglass layers are built up behind it. Unlike paint, gel coat is integral to the boat's construction and becomes the outermost surface when the part releases from the mold.
Gel coat is created by mixing polyester resin with pigments for color, plus additives like UV stabilizers, thixotropic agents (to control flow), and catalysts that trigger curing. The formula determines everything from color consistency to how well it resists UV degradation.
During boat manufacturing, gel coat is sprayed directly into the mold. The mold surface is polished to a mirror finish, and the gel coat takes on that smoothness. Once the gel coat cures to a tacky state, fiberglass mat and resin are applied behind it. When the finished part releases from the mold, the gel coat becomes the exterior surface.
Key point: Gel coat isn't a coating applied to a finished boat. It's manufactured into the boat from the start. This is why repairs and restoration work differently than automotive paint.
How thick is gel coat on a boat?
Gel coat thickness typically ranges from 15 to 25 mils (0.015 to 0.025 inches), roughly the thickness of a few sheets of paper. Quality manufacturers target 18-20 mils, but thickness varies across the boat -- complex curves and edges are often thinner than flat surfaces. This matters for restoration because aggressive polishing or sanding removes gel coat material.
Gel coat thickness typically ranges from 15 to 25 mils (0.015 to 0.025 inches). That's roughly the thickness of a few sheets of paper. Quality manufacturers target 18-20 mils for consistent coverage.
Thickness varies across the boat. Complex curves, tight corners, and vertical surfaces often end up thinner than flat horizontal areas. I've worked on boats where the gel coat on the hull sides measured 22 mils while the gunwale edges measured only 12 mils.
This matters for restoration work. When compounding or wet sanding to remove oxidation, you're removing gel coat. On areas that started thin, you can burn through to the fiberglass faster than expected. A paint thickness gauge helps, but experience matters more. The gel coat changes appearance slightly as you approach the fiberglass beneath.
Why does gel coat fade, chalk, and degrade over time?
Gel coat degrades primarily from three factors: UV radiation breaks down the polyester resin's molecular bonds (causing chalky oxidation), salt and environmental exposure create micro-abrasion and moisture damage, and lack of protective products like wax, sealant, or ceramic coating allows the built-in UV stabilizers to deplete within 2-3 years.
Gel coat is tough, but it's fighting a losing battle against three main enemies:
UV radiation
Ultraviolet light breaks down the molecular bonds in polyester resin. The surface layer degrades first, creating that chalky, oxidized appearance. This process is called photodegradation, and it happens faster on boats because water reflects additional UV back onto the hull.
On the Chesapeake Bay and around Ocean City, boats get hit with intense summer sun. A boat without UV protection can show visible oxidation in a single season.
Salt and environmental exposure
Salt doesn't directly attack gel coat the way it corrodes metal. But salt crystals are abrasive, and they attract moisture. Dried salt on gel coat creates microscopic scratches during washing if you're not careful. Salt spray also leaves deposits that can etch into the surface over time.
Lack of protection
Gel coat has some UV resistance built in, but it's not self-sustaining. Manufacturers expect that owners will maintain the surface with wax, sealant, or ceramic coating. Without that protection, the UV stabilizers in the gel coat deplete within 2-3 years, and degradation accelerates.
How do you know if your boat's gel coat needs repair or restoration?
Five key signs indicate gel coat needs attention: chalky white residue when you rub the surface, faded color compared to protected areas, water no longer beading on the surface, stains that won't wash off because they have penetrated the porous oxidized layer, and a rough texture that persists even after washing.
- Chalky residue when you rub your hand across the surface. This is oxidized resin.
- Faded color compared to protected areas under hardware or graphics.
- Water doesn't bead anymore. Instead, it sheets or just sits flat.
- Stains won't wash off because they've penetrated the porous oxidized surface.
- Surface feels rough even after washing.
What is the best way to maintain gel coat on a boat?
Four maintenance habits extend gel coat life by decades: rinse with fresh water after every use, wash with pH-neutral marine soap (never dish soap), maintain consistent UV protection (wax lasts 2-4 weeks, sealant 3-6 months, Thor ceramic 12-18 months, Kraken ceramic up to 24 months), and address oxidation early before it requires aggressive correction.
Proper maintenance extends gel coat life by decades. Here's what actually works:
Rinse after every use
Fresh water removes salt before it dries. This single habit prevents more damage than anything else.
Wash with proper products
Use a pH-neutral marine soap. Dish soap strips existing protection. Harsh chemicals accelerate UV damage.
Maintain UV protection
Wax lasts 2-4 weeks in the marine environment. Polymer sealant lasts 3-6 months. Thor ceramic lasts 12-18 months; Kraken ceramic lasts up to 24 months. Pick one and stay consistent.
Address oxidation early
Light oxidation comes off with polish. Moderate oxidation needs compound. Heavy oxidation requires wet sanding. The longer you wait, the more aggressive the correction.
Frequently asked questions
What is gel coat made of?
Gel coat is made from polyester resin mixed with pigment for color. It also contains UV stabilizers and other additives to improve durability. During manufacturing, it's sprayed into a mold before the fiberglass layers are applied, so it becomes the outermost surface of the finished boat.
How thick is gel coat on a boat?
Gel coat thickness typically ranges from 15 to 25 mils (0.015 to 0.025 inches). Some areas may be thinner due to manufacturing variations, especially on complex curves or edges. This matters for restoration because aggressive polishing or sanding can remove gel coat faster than expected in thin areas.
Why does gel coat turn chalky?
Gel coat turns chalky due to oxidation caused by UV exposure. Ultraviolet rays break down the molecular bonds in the polyester resin, causing the surface to degrade. The chalky residue you see is actually degraded resin particles. This process accelerates without proper UV protection like wax, sealant, or ceramic coating.
Can oxidized gel coat be restored?
Yes, oxidized gel coat can usually be restored through compounding and polishing. Light oxidation comes off with polish alone. Heavy oxidation may require wet sanding followed by multiple compounding and polishing steps. Once restored, the gel coat should be protected with ceramic coating or sealant to prevent future oxidation.
What is the difference between gelcoat and paint?
Gel coat is a polyester resin applied during manufacturing as the outermost layer of a fiberglass boat. It's part of the hull structure, not a coating added on top. Marine paint like Awlgrip or Awlcraft 2000 is a urethane-based finish applied over a primer, similar to automotive paint. Gel coat is thicker (15-25 mils) and more porous. Paint is thinner, harder, and easier to repair. Most production boats use gel coat. Custom sportfish and yachts often use painted finishes.
What is the difference between fiberglass and gelcoat?
Fiberglass is the structural material that makes up the hull. It consists of layers of glass fiber fabric bonded together with resin. Gel coat is the thin, colored outer surface sprayed into the mold before the fiberglass layers are laid up. Think of gel coat as the skin and fiberglass as the skeleton. Gel coat provides color, UV protection, and a smooth surface. Fiberglass provides the strength. When gel coat wears through from sanding or severe neglect, you see the raw fiberglass beneath.
Can you paint directly over gelcoat?
Yes, but the gel coat must be properly prepared first. Sand the surface to create mechanical adhesion (typically 320-400 grit), clean with solvent to remove all contaminants, apply a compatible primer, then paint. Skipping the primer or sanding is the most common reason paint fails on gel coat. Marine urethane paints like Awlcraft 2000 or AwlGrip produce excellent results over prepped gel coat. This is typically done when gel coat is too far gone for correction to restore.
Is gelcoat just epoxy?
No. Gel coat is made from polyester resin, not epoxy. They are different chemistries. Polyester resin cures through a chemical reaction with a catalyst (MEKP) and is more flexible and UV-resistant than epoxy. Epoxy is stronger in adhesion and moisture resistance but yellows in UV exposure and is more brittle. Gel coat uses polyester because it needs to flex with the hull and resist UV degradation for years. Epoxy is used in structural repairs and barrier coats below the waterline.
Why does gel coat maintenance experience matter for detailers?
Gel coat behaves differently than automotive paint. It is more porous, varies in thickness across the hull, and requires specific compounds and machine settings. A detailer with manufacturing or marine-specific background understands how gel coat is built, where it runs thin, and how to correct oxidation without burning through to the fiberglass beneath.
Having built custom sport fishing boats at Jarrett Bay and Duffie Boatworks, I came to gel coat from a different angle. Custom sportfish use Awlgrip and urethane paint finishes - more forgiving than gel coat, easier to repair. Most boats I detail now have gel coat, which behaves completely differently. Running boats as a captain showed me how gel coat oxidizes over seasons of hard use. Starke Yacht Care certification taught me the specific correction techniques. That combination shapes how I approach every job.
The most common mistake I see is neglect followed by aggressive correction. Owners let oxidation build for years, then want it fixed in one session with heavy compounds. Sometimes that works. Sometimes we burn through thin spots trying. The better approach is consistent maintenance that never lets the oxidation get severe.
Gel coat is your boat's first line of defense. Treat it right, and it'll protect the fiberglass beneath for 30+ years. Neglect it, and you're looking at expensive repairs or a full repaint.
Need gel coat restoration?
I'll assess your gel coat condition and recommend the right approach for your boat.