I have heard references to gelcoat being a water proof application for a fiberglass boat hull. I have always considered fiberglass as inherently water proof. There are several industries surrounding fiberglass as a roofing product.

I know gelcoat serves several purposes, most notably it is smooth and UV resistant.

From a boat prospective; The gelcoat is usually applied to the mold, with the fiberglass applied next. To the best of my knowledge no one applies a layer of gelcoat to the inside of the hull. Which means that big fiberglass boat hull is an untreated bucket of water, it is protected on the outside from by the gelcoat, but nothing is protecting the inside. I would imagine in most cases, there is always some standing water someplace in most boats, most drains I have seen leave at least a cup of water in the boat, and I can't say how many I have seen collecting rain water.

What if any damage might soaking fiberglass in water do to the fiberglass?

  • Isn't the gelcoat more about making the surface smooth to enable it to have less drag and less about making it more waterproof (not a sailor so this is a bit of a guess)
    – user2766
    Oct 24, 2016 at 13:10
  • Also isn't about UV protection? I know most plastics break down under UV light
    – user2766
    Oct 24, 2016 at 13:11
  • "To the best of your knowledge no one applies a layer of gelcoat to the inside of the hull." Do you have any pictures of raw fiberglass on the top or bottom sides of a fiberglass boat? Protection is not limited to gel coat.
    – paparazzo
    Oct 24, 2016 at 13:29
  • @Paparazzi yes, my2fish.files.wordpress.com/2009/10/… from here my2fish.wordpress.com/2009/10/10/i-cut-2-holes-in-my-sunfish which looks like an interesting article Oct 24, 2016 at 13:42
  • OK that looks like raw fiber glass to me. I have always seen sealed.
    – paparazzo
    Oct 24, 2016 at 13:45

1 Answer 1


Often when we say "Fibreglass" we're actually referring to a composite of both fibreglass and resin (usually polyester for glass, epoxy for carbon/kevlar - sometimes glass). This is important, that resin is a distinct component was missing from the question.

The composite is formed when glass fabric is shaped in/on a mold and saturated with resin. The resin then cures, forming a rigid structure.

Is this waterproof? Yes. If the resin completely surrounds the fibre.

In production (after initial curing), loose strands will often have to be sanded down and additional resin applied to ensure a compete seal. If, through faulty manufacture or damage, glass is exposed to water (especially salt-water) this will cause to composite to break apart/de-laminate.

So why gelcoat? To protect the glass/resin composite, primarily from UV. Exposing resin to UV will cause it to degrade and crack over time, eventually exposing the fibres to the elements.

Gelcoat has other benefits - it's often more durable and a convenient way of adding a coloured layer. Gelcoat can be applied directly to the mold to form a convenient and better bonded composite, but can also be applied later.

So why doesn't it matter if gelcoat is used on the inside? As above, if the glass is sealed properly with resin, the inside of the boat won't need the extra protection from a gelcoat.

Note: fibreglass roofing should have a clear/colourless gelcoat for UV protection, but of course manufacturers will make whatever they're paid to make.

  • To add, fiberglass construction also usually consists of a sandwich of fiberglass layers surrounding a core of wood, foam, or other materials. If water gets into the core it can begin rotting it, or cause freezing damage, or other problems. Oct 24, 2016 at 23:02
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  • @JamesJenkins that link refers to polyester resins. Polyester is much less water resistant than epoxy but due to prices is still widely used for big productions, lots of hulls are built with it. Epoxy resins do not suffer hydrolysis like polyester and vinylesters, it still costs quite a bit more. Nov 2, 2016 at 18:56
  • Some polyester laminations need to have a way to "breathe" thats why they are not coated inside and out. Polyester is somewhat porous, can suffer from osmosis even with good manufacturing processes. The inside of a boat is simply not supposed to be constantly in water like the outside but if you take even the best made polyester boat and you do not maintain it properly the fiberglass will "rot" from inside out, quite a few that neglected bilge maintenance because "the boat is made of plastic and not wood" found that surprise Nov 2, 2016 at 19:10

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