A steady stream of small bubbles rising through an otherwise well-mixed pour usually isn’t coming from your technique at all — it’s coming from the substrate itself, releasing air that was trapped there long before you ever opened the epoxy.
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, ruining the smooth finish, particularly in thicker coats. This is almost always triggered by the exothermic heat of the curing epoxy or a rise in ambient temperature.
Why Concrete and Porous Substrates “Exhale”
Porous materials are filled with microscopic air pockets, and two factors cause that trapped air to expand and escape once epoxy is applied. First, the exothermic reaction between resin and hardener generates heat that warms the substrate, expanding the air and moisture trapped in its pores. Second, that expanding air raises pressure within the substrate, forcing bubbles through the wet epoxy film to the surface, where they pop and often leave permanent craters or pinholes if the epoxy has already begun to gel. Concrete adds a third factor: trapped moisture that, as the epoxy begins to cure and cross-link and attempts to seal the surface, increases in vapor pressure and forces itself out as bubbles.
Genuine Solutions for Controlling Outgassing
The solution is to seal the substrate before applying the final flood coat, so trapped air never reaches the thick, visible layer.
1. The Essential Seal Coat Strategy
Apply a thin, transparent seal coat of the same epoxy resin before the main pour — thin enough to penetrate surface pores and act as a pore blocker. Immediately after applying it, use a flat squeegee or roller to push material into the surface, then pass a heat gun or torch quickly over it to pop any initial rising bubbles; this purging step works because the seal coat is thin enough to let air escape easily. Let the seal coat cure until tacky to the touch but no longer liquid, usually 4 to 12 hours depending on product and temperature — the pores are blocked, but the surface stays chemically active enough to bond fully with the final flood coat.
2. Concrete-Specific Preparation and Moisture Control
Always test concrete slabs with a calcium chloride test kit or an electronic moisture meter — if the moisture vapor emission rate or relative humidity is too high, the epoxy will fail regardless of sealing. Where moisture levels run high, a specialized moisture-tolerant epoxy primer, formulated to chemically bond to damp concrete and form a complete vapor barrier, addresses the problem at its source. Diamond grinding the concrete to achieve a proper surface profile removes weak, dusty, or contaminated surface laitance and also helps expose and relieve some of the trapped air before the pour even starts.
3. Environmental and Timing Control
Concrete acts like a heat sink, so as ambient temperature rises through the day, the slab warms and trapped air expands, pushing outgassing into whatever coat is applied at that moment. Begin the seal coat and main pour when ambient temperature is stable or slightly decreasing, such as late afternoon or evening, which keeps internal air pressure lower than the outside pressure and minimizes the push of trapped air. Keep the work area temperature consistent within the manufacturer’s recommended range throughout the entire cure, avoiding drafts or rapid changes that could trigger sudden substrate expansion.
Wood substrates outgas by a similar mechanism but with an added variable: species and grain structure. Open-pore woods like oak or ash have larger, more interconnected air channels than tight-grained woods like maple, so they tend to outgas more aggressively and for longer once heat is applied, even at the same moisture content. End grain is particularly prone to this, since it exposes the open ends of the wood’s vascular structure directly to the epoxy rather than the tighter face-grain surface. A seal coat brushed specifically into end-grain sections, rather than just rolled evenly across the whole piece, catches a source of bubbles that an otherwise well-executed seal coat can still miss.
Incure’s industrial floor-coating and potting formulations account for outgassing risk directly in their application instructions, since a moisture-emission problem on a concrete substrate produces the identical defect whether the coating is decorative or functional. A simple plastic-sheet moisture test, taped down overnight and checked for condensation the next morning, catches most outgassing-prone slabs before a single batch of coating is ever mixed. Email Us if a concrete moisture test comes back borderline and you need help selecting a compatible primer.
See also how CTE mismatch causes adhesive bond failure for a related substrate-interaction problem, and choosing a UV lamp for resin curing if your seal-coat process also involves light-cure steps. Contact Our Team for guidance on a persistent outgassing problem on a specific substrate.
Visit www.incurelab.com for more information.