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How Fiberglass Pools Are Made, Part 3 Osmotic Blister Protection

Updated: Mar 16, 2021

In the last article, we covered all of the critical steps to creating a long-lasting and durable gel-coat finish on your fiberglass pool. In this article, we will cover the first layer of fiberglass right behind the gel coat which really determines the lifetime integrity of your pool's Gelcoat by preventing osmotic blisters.

For decades Osmotic Blisters were significant in the fiberglass manufacturing industry. Particularly in boats and fiberglass pools. However, advancements in materials, technologies, and best practices have evolved so that it doesn't have to a problem any longer.

In today's article, we are going to address 3 big questions: 1: What is an Osmotic Blister and how does it occur. 2: How are they prevented 3: How do you know your pool is protected?

Osmotic Blisters
Osmotic Blister


An osmotic blister is exactly what it sounds like. A blister in the gel coat of a fiberglass pool or boat, etc.

An osmotic blister occurs when water that flows through the Gelcoat is allowed to penetrate into the fiberglass behind the Gelcoat. This creates a small pocket that builds enough pressure to create a bulge and push out the gel coat surface. This is an osmotic blister. They occur because the first layer of fiberglass and resin behind the gelcoats were not waterproof. Polyester resin is not waterproof yet for decades was used throughout the fiberglass manufacturing industry and was the culprit that caused blistering.

Osmotic Blister Protection with Vinyl Ester Resin
Vinyl Ester Resin Blister Protection


This one is easy. Use the correct resin for the first layer of fiberglass right behind the Gelcoat. This resin must be 100% waterproof so as not to allow water to seep into the fiberglass layer behind the Gelcoat. This type of resin is 100% Vinyl Ester Resin. In decades past, polyester resin was used as the first layer behind the gel coat. Polyester resin is not waterproof which is the reason osmotic blisters were an issue. Most modern fiberglass manufacturers have realized this and made the conversion to the waterproof vinyl ester resin.


Read the fine print! You will be able to tell if your pool is protected against Osmotic Blistering by a clearly written warranty. River Pools warranty clearly states that the pool is protected from Osmotic Blistering for 15 years. If your warranty reads the same you will know that your pool is built with vinyl ester resin as the first layer after the Gelcoat and that osmotic blisters will not be a worry for your pool.

In this video, Cristian goes into great detail to show how the River Pools Way of Manufacturing provides a set of best practices to protect your pool from osmotic blisters.

If you are ready to catch the wave, like we have, call Pool Pros today at 920-771-0107, email us at or check out our fiberglass pool models here.


How Fiberglass Pools are Made Part 1 The Mold

How Fiberglass Pools are Made Part 2 The Gelcoat

How Fiberglass Pools are Made Part 4 Polyester Chop

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