How to Spray Gelcoat: Protecting Your Fiberglass Boat

Gelcoat is the most common fiberglass boat finish. In fact, almost 90% of fiberglass boats new from the factory come with a gelcoat finish. Gelcoat has a somewhat controversial reputation in boating, but once you’ve decided to stick with a gelcoat instead of paint, you’ll soon realize you have to retreat and repaint your gelcoat on occasion.

Respraying gelcoat is an intensive process that should be undertaken by a professional. The surface preparation is very similar to the surface preparation needed for painting, including surface repair of dings and the application of a primer. The application process itself is very different though.

Gelcoat sprayings do not level out and cure evenly the way an ideal paint job does. As such, in between each spraying, you will need to sand the surface of the gelcoat to remove 5-10 mils of thickness before you spray on the next coat. Over time, your gelcoat will oxidize quicker than paint and require polishing. Your gelcoat loses around 3-5 mils of thickness per year of use. A well-maintained gelcoat should last around 8 to 10 years of use. But this requires constant maintenance and polishing.

Before you spray your gelcoat, you must thin it out and use a large spray tip (2mm & 2.2 mm HVLP gravity feed cup guns are best). The less you thin your gelcoat, the better for long-term coverage.

Your first coat should be between 30-35 mils thick. Once you have sprayed the coat and its fully dry, you want to sand off approximately 5-10 mils of thickness. If you leave your initial coat at a thickness greater than 25 mils, your coat will suffer from cracks over time simply because the coating will be too thick.

How to Spray Gelcoat

How to Spray Gelcoat

After the initial coating and sanding, you will need to spray on 4-5 more coats at a thickness of approximately 10 mils each. These subsequent coats will be much thinner than your initial coat. In between coats, you should allow the surface to harden so that you can’t put an impression into it with your fingernail or a dull knife. The first two coats will have slight tackiness, but by your third coat, you should notice no tackiness once the surface is hardened.

After your third coat, sand the surface with a 180 grit sandpaper so the surface loses its orange peel look and you are left with a smooth finish. Finally, the last coat of gelcoat will have certain additives mixed in and should be applied with a 1 mm spray tip. Most boaters add around 30-40% patch booster and 10% surfacing wax, which thins the gelcoat quite a bit. When spraying the last coat, a good technique is to spray 1-2 mils at a time and then going back over that area after a few minutes.

Allow the final coat to fully cure- anywhere from 24 hours to several days- before sanding it with 1000 grit sandpaper. Wash the dull finish and then finish it with a finish sealant and a swirl remover. After you’ve completely cured this layer, you’ll have a fully protecting gelcoat that will last for years with regular maintenance.

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