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Understanding Thermal Shock When Fiberglassing Surfboards

Understanding Thermal Shock When Fiberglassing Surfboards

Fiberglassing surfboards involves applying a resin (a type of liquid plastic) to the surfboard to make it strong and smooth. However, if the resin and the surfboard blank (the foam core of the surfboard) are at different temperatures, it can cause thermal shock, leading to problems like uneven curing, orange peel texture, cracks, and air bubbles. Let's explore what thermal shock is, how it happens, and how to avoid it.

What is Thermal Shock?

Thermal shock happens when there is a sudden change in temperature, which can stress and damage materials. When you're fiberglassing a surfboard, thermal shock occurs if you apply warm resin to a cold surfboard blank. This sudden temperature difference can cause various issues that affect the surfboard's quality.

How Does a Cold Surfboard Blank Affect Warm Resin?

When warm resin is applied to a cold surfboard blank, several problems can occur:

  1. Uneven Curing: The resin doesn't harden evenly because the cold parts of the blank slow down the curing process while the warmer areas cure faster. This results in a weak and inconsistent finish.

  2. Orange Peel Texture: The surface of the resin can develop a bumpy, uneven texture similar to an orange peel because the resin doesn't level properly due to the temperature difference.

  3. Cracks and Delamination: The stress from thermal shock can cause cracks and separation in the resin layer. As the resin and blank expand and contract at different rates, the bond between them weakens.

  4. Air Bubbles: The temperature difference can cause air bubbles to form in the resin, leading to weak spots and a compromised finish.

Best Temperature Conditions for Fiberglassing

To avoid thermal shock, it's essential to ensure that both the surfboard blank and the resin are at the same temperature before starting. Here are some tips:

  1. Equalize Temperatures: Store the surfboard blank and resin in the same room for several hours before starting the fiberglassing process. This allows both materials to reach the same temperature.

  2. Control the Workspace Temperature: Aim for a consistent workspace temperature between 70°F and 85°F (21°C and 29°C). This range is ideal for the curing process and helps ensure a smooth, even finish.

  3. Use a Thermometer: Monitor the temperature of your workspace, surfboard blank, and resin with a thermometer to maintain consistent conditions.

  4. Pre-Warm the Blank: If your workspace is cold, consider warming up the surfboard blank to match the temperature of the resin using space heaters or heat lamps, but be careful not to overheat it.

Dealing with High Temperatures

When temperatures are high, it's best to fiberglass in the evening when temperatures are falling. This helps prevent the surfboard blank from releasing trapped air (outgassing) as it heats up. Outgassing causes air bubbles to rise through the curing resin. Here's how to manage high temperatures effectively:

  1. Glass in the Evening: Plan your fiberglassing sessions for late afternoon or evening when temperatures start to drop to minimize the risk of outgassing and air bubbles.

  2. Monitor Temperature Trends: Start the fiberglassing process as the temperature begins to fall, rather than during the hottest part of the day.

  3. Use Fans or Ventilation: Enhance airflow in your workspace with fans or proper ventilation to maintain a stable temperature and reduce the risk of overheating.

  4. Avoid Direct Sunlight: Keep your workspace shaded and avoid direct sunlight, which can cause rapid temperature increases and outgassing.

The Role of Wood Stringers

Wood stringers are strips of wood that run down the center of many surfboards to provide strength and stability. However, wood has a different thermal mass compared to foam, which can lead to specific challenges:

  1. Different Thermal Mass: Wood absorbs and retains heat differently than foam. This can cause uneven temperature distribution and exacerbate thermal shock issues.

  2. Air Bubbles at the Intersection: When the temperature rises, the wood stringer can cause localized outgassing at the intersection of wood and foam, leading to air bubbles.

  3. Pre-Warm Stringers: Ensure that wood stringers are pre-warmed to the same temperature as the foam blank to reduce temperature disparities and minimize air bubbles.

  4. Apply Resin Evenly: When applying resin over wood stringers, take extra care to ensure an even coating. Use a brush or roller to work the resin into the stringer and foam intersection, reducing the likelihood of trapped air.

Additional Tips for Successful Fiberglassing

To achieve a high-quality finish on your surfboard, follow these additional tips:

  1. Use High-Quality Materials: Invest in high-quality resin, fiberglass cloth, and other materials. Poor-quality products can worsen thermal shock issues and lead to subpar results.

  2. Mix Resin Thoroughly: Properly mix the resin and hardener according to the manufacturer's instructions to ensure even curing. Maine Grade Epoxy Resin is 2 parts resin to 1 part hardener.

  3. Apply Resin in Thin Layers: Apply resin in thin, even layers to ensure a smooth finish. Thick layers are more prone to trapping air bubbles and developing an orange peel texture.

  4. Use a Good Resin Spreader: Spread the resin evenly across the surface of the blank using a good resin spreader to work out any air bubbles and ensure a uniform coat.

  5. Monitor Humidity Levels: High humidity can affect the curing process of epoxy resin. Aim for a workspace with humidity levels between 40% and 60% for optimal results.


Thermal shock is a significant concern when fiberglassing surfboards, especially when using warm resin in a cold workspace. By understanding the science behind thermal shock and implementing best practices for temperature control, you can achieve a smooth, durable finish on your surfboards. Make sure both the surfboard blank and resin are at the same temperature before starting the process. Consider glassing in the evening when temperatures are falling to prevent outgassing. Pay special attention to wood stringers, as their different thermal mass can lead to localized issues. By following these guidelines, you can minimize the risks associated with thermal shock and create high-quality, long-lasting surfboards.

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