If you’ve finished preparing a concrete slab and notice imperfections after grinding, you might be dealing with pinholes. These small voids can lead to bubbles, uneven textures, or other surface issues. Knowing what causes them and how to manage them is important for anyone using APC systems like Ultra Flake® and Hyper Flake®.
What Are Pinholes in Concrete?
Before applying any coating, it’s important to know what pinholes are and why they form in concrete slabs.
Pinholes are tiny holes or voids formed in a concrete slab during pouring and curing. Often hidden below the surface, they become visible only after grinding or surface preparation.
Concrete absorbs and releases air and moisture like a sponge. Pinholes amplify this process, allowing trapped air to escape through coatings and potentially causing bubbles while it’s curing.
Key points applicators need to know:
- Softer, more porous slabs are more prone to pinholes, but dense slabs can develop them too.
- Strength and porosity can vary across a slab, meaning some areas may be more susceptible.
- Slabs exposed to heat, direct sunlight, or over-preparation are especially vulnerable.
For more guidance on preparation, watch our video Reading a Concrete Slab.
How Pinholes Cause Bubbles in Concrete Coatings
Pinholes act like tiny bowls in the concrete, letting trapped air escape through coatings. The effect depends on the coating’s stage of cure.
When working with APC systems such as Ultra Flake® or Hyper Flake®:
1. Wet coatings
Air releases smoothly, and bubbles generally settle back into the film because the coating still has enough flow to self-level.
2. Partially cured coatings
Once the coating begins to set, escaping air can leave rings, craters, or raised spots, commonly seen when epoxy primers are past their ideal recoat window.
3. Fully cured coatings
Once fully cured, the coating cannot flow to close defects. Trapped air escaping from below can create pronounced bubbles, often giving a ‘bubble wrap’ effect, especially in fast-curing topcoats.
Porous slabs and high temperatures increase bubble risk, affecting both finish quality and workflow.
How Seasonal Changes Affect Pinholes in Concrete
Temperature swings have a major impact on pinhole formation and outgassing. Summer heat naturally increases the risk, but winter conditions in Australia can also present challenges.
Even in winter, slabs can heat 20-25°C from morning to midday. This expansion and contraction can release air, causing bubbles if coatings are applied at the wrong time.
APC systems are formulated for Australian conditions, and being Australian Owned & Manufactured ensures our products are developed with local climate patterns in mind. This supports consistent results across seasonal changes, though applicators should still monitor surface and ambient temperatures closely to maintain finish quality.
Practical Tips for Applicators to Minimise Concrete Pinholes
Reducing pinholes requires preparation, timing and environmental control. Here’s APC’s applicator-focused advice:
Avoid direct sunlight on slabs.
Sunlight accelerates the expansion of air and moisture in concrete, increasing bubble risk. Apply coatings in shaded areas, indoors, or use temporary covers to protect the slab from direct heat.Schedule your application strategically.
Apply coatings after the hottest part of the day or before temperatures rise, typically before 7 am and after 3 pm in most Australian cities. This helps minimise outgassing and ensures smoother finishes.Control the slab and ambient temperature.
Maintain consistent slab and air temperatures using shade, temporary coverings, or ventilation. APC’s Australian Made coatings are designed for local conditions, but controlling environmental factors is essential for predictable results.Adjust for extreme weather.
High heat, cold snaps, or sudden weather changes can dramatically increase outgassing. When extreme conditions are expected, reschedule your application to protect the finish and workflow.Follow the APC system best practices.
- Surface preparation: Grind slabs with appropriate diamond pads (e.g., 40–60 grit) and vacuum thoroughly.
- Application technique: Use applicator-friendly methods to spread coatings evenly, reducing the likelihood of bubbles over pinholes.
- Product choice: Apply Ultra Flake® or Hyper Flake® systems as per APC guidelines, benefiting from coatings formulated for Australian climates and conditions.
Following these tips helps flooring and epoxy applicators achieve smooth, professional finishes, minimise defects and maintain consistent workflow, reinforcing why APC systems are trusted across Australia. See more APC Insights for applicators here.
FAQs: Pinholes and Concrete Coatings
Pinholes form during curing when air gets trapped in the slab. They become problematic when coatings are applied over these tiny voids.
Yes. Even high-strength, dense slabs can develop pinholes, especially in areas exposed to heat, sunlight, or over preparation.
Pinholes allow trapped air to escape through your coating, creating bubbles that compromise finish and texture.
By controlling environmental conditions, scheduling applications correctly, shading the slab, and following APC’s system application guides.
Absolutely. Both summer heat and winter temperature swings affect slab expansion and outgassing, increasing the risk of bubbles.
Why Understanding Pinholes Matters for Applicators
Pinholes in concrete can be challenging, but understanding slab behaviours, temperature effects, and outgassing helps flooring and epoxy applicators:
- Minimise bubble formation in APC Broadcast and High Build Systems.
- Achieve smooth, professional finishes for clients.
- Maintain consistent workflow and avoid costly rework.
Understanding pinholes and their impact on coatings is key to achieving smooth, professional finishes. By monitoring slab conditions, controlling environmental factors and following APC’s system guidelines, applicators can minimise bubbles and surface defects, ensuring high-quality results every time.
Have Questions About Epoxy?
Want to learn more about concrete preparation and how to handle pinholes on-site? Watch our video: Concrete Preparation: What Are Pinholes? for step-by-step guidance from the experts at All Purpose Coatings.
What Causes Pinholes in Concrete? Expert Tips for Epoxy Flooring Applicators