Outgassing is the process where air or moisture trapped within a porous substrate—such as concrete, wood, or stone—is released into the liquid epoxy coating. This is a common and frustrating problem that causes a continuous stream of small bubbles and voids, ruining the smooth finish, particularly in thicker coats.

This phenomenon is almost always triggered by the exothermic heat of the curing epoxy or a rise in ambient temperature.

Why Concrete and Porous Substrates “Exhale”

Outgassing occurs because porous materials are filled with microscopic air pockets. When the epoxy is applied, two factors cause the trapped air to expand and escape:

1. Exothermic Heat: The chemical reaction between the epoxy resin and hardener generates heat. This heat warms the substrate, causing the air and moisture trapped inside its pores to expand.
2. Pressure Differential: The expanding air increases pressure within the substrate, forcing the air bubbles to push through the wet, liquid epoxy film to the surface, where they pop and often leave permanent craters or pinholes if the epoxy has begun to gel.
3. Hydrostatic Pressure (Concrete): Concrete, in particular, can contain trapped moisture. As the epoxy begins to cure and cross-link, it attempts to seal the surface, which can increase the vapor pressure of the moisture inside the concrete, forcing it to bubble out.

Genuine Solutions for Controlling Outgassing

The solution is to seal the substrate before applying the final flood coat, preventing the air from escaping into the thick, visible layer.

1. The Essential Seal Coat Strategy

• Apply a Thin Seal Coat: Before the main, thick pour, apply a very thin, transparent seal coat of the same epoxy resin. This coat should be thin enough to penetrate the surface pores and act as a pore blocker.
• Manage the Initial Bubbles: Immediately after applying the seal coat, use a flat squeegee or roller to push the material into the surface. Then, pass a heat gun or torch quickly over the surface to pop any initial bubbles that rise. This purging process is essential, as the seal coat is thin enough to allow the air to escape easily.
• Allow to Partially Cure: Let the seal coat cure until it is tacky to the touch but no longer liquid (usually 4–12 hours, depending on the product and temperature). This means the pores are blocked, but the surface is still chemically active enough to bond perfectly with the final flood coat (intercoat adhesion).

2. Concrete-Specific Preparation and Moisture Control

• Test for Moisture: Always use a calcium chloride test kit or an electronic moisture meter on concrete slabs. If the concrete’s moisture vapor emission rate (MVER) or relative humidity (RH) is too high, the epoxy will fail.
• Use a Moisture Mitigation Primer: If moisture levels are too high, a specialized moisture-tolerant epoxy primer must be applied first. These primers are formulated to chemically bond to damp concrete and create a complete vapor barrier, stopping the outgassing caused by moisture.
• Diamond Grinding: Ensure the concrete is properly prepared (usually via diamond grinding) to achieve a proper surface profile (CSP 2-3) and remove any weak, dusty, or contaminated surface laitance. This also helps expose and relieve some of the trapped air.

3. Environmental and Timing Control

• Pour on a Falling Temperature Schedule:This is critical for concrete. Concrete acts like a heat sink. As the ambient temperature rises (e.g., in the morning), the concrete warms up, causing the trapped air to expand and outgas.
  • Solution: Begin your seal coat and main pour when the ambient temperature is stable or slightly decreasing(e.g., late afternoon or evening). This keeps the air pressure inside the concrete lower than the air pressure outside, minimizing the “push” of trapped air.
• Maintain Consistent Temperature: Keep the work area temperature consistent and within the manufacturer’s recommended range throughout the entire cure. Avoid drafts or rapid temperature changes that could cause sudden expansions in the substrate.