The most effective prevention is to prepare and seal the concrete correctly, apply epoxy under suitable temperature conditions and avoid introducing excessive air during mixing and application.

What Is the Difference Between Bubbles, Pinholes and Craters?

Although these defects may look similar, they can have different causes and require different corrective actions.

Defect	Appearance	Common Cause
Bubbles	Raised, rounded pockets in the coating	Trapped air, outgassing or application-related air entrainment
Pinholes	Small openings or holes in the cured coating	Air escaping through wet epoxy or bubbles bursting before cure
Craters or fish eyes	Circular depressions where the coating pulls away	Surface contamination or coating rejection
Blisters	Raised areas or pockets beneath a coating film	Moisture, trapped materials or adhesion-related problems

Correctly identifying the defect is important because surface bubbles, coating rejection and moisture-related blistering should not automatically be treated in the same way.

For contamination-related defects, see Why Does Floor Coating Reject or Form Fish Eyes?.

What Causes Bubbles and Pinholes in Epoxy Floors?

1. Concrete Outgassing

Concrete contains pores and small voids that can hold air. As the concrete temperature increases, trapped air can expand and escape through freshly applied epoxy.

If the coating begins to cure before the air escapes completely, bubbles or small openings may remain in the finished surface.

Outgassing is particularly common when:

Epoxy is applied to concrete that is warming rapidly.
Direct sunlight heats the concrete during application.
The substrate contains numerous pores or small voids.
The concrete has not been adequately sealed before coating.
A thick epoxy layer is applied directly to porous concrete.

How to Prevent Concrete Outgassing

Where possible, apply epoxy while the concrete temperature is stable or gradually falling rather than rapidly increasing.

Monitor the substrate temperature rather than relying only on ambient air temperature. Concrete exposed to sunlight can be considerably warmer than the surrounding air.

Proper priming also helps seal surface pores and reduce the release of trapped air into subsequent coating layers.

2. Porous or Inadequately Primed Concrete

Highly porous concrete can absorb epoxy unevenly and release air during application. If the first coat does not adequately seal the substrate, bubbles and pinholes may continue to appear in subsequent coats.

This is particularly relevant to older concrete, mechanically prepared porous surfaces and concrete containing numerous small voids.

A suitable epoxy primer should wet and seal the prepared substrate before the finish coats are applied.

Choosing a Fortis Epoxy Primer

Concrete Condition	Fortis Option	Purpose
Dry, porous concrete	Fortis 823 Prime	Penetration, sealing and rapid recoating
Poor, porous or difficult concrete	Fortis 820 Prime	Substrate wetting, adhesion and consolidation
Sound concrete suitable for self-priming	Fortis AquaCoat or Coat 823 HS, where specified	Compatible self-priming flooring system
Significant moisture transmission	Fortis HydroFort VB, where suitable	Moisture vapour control rather than ordinary priming

Highly absorbent concrete may require additional wet-on-wet primer application to obtain a uniformly sealed surface, subject to the selected primer's Technical Data Sheet.

For a more detailed comparison, see How to Choose an Epoxy Floor Primer.

3. Air Introduced During Epoxy Mixing

Two-component epoxy products must be thoroughly mixed to achieve a uniform cure. However, incorrect mixing techniques can introduce excessive air into the material.

High-speed mixing, whipping the material or repeatedly lifting the mixing blade above the liquid surface can create bubbles that remain in the coating after application.

To minimise air entrainment:

Use a suitable low-speed mechanical mixer.
Keep the mixing blade submerged during mixing.
Avoid whipping or creating a vortex in the material.
Incorporate material from the sides and bottom of the container.
Follow the mixing procedure specified in the Technical Data Sheet.
Mix only quantities that can be applied within the working time.

Allowing mixed material to stand briefly may help release entrained air where the product instructions permit, but this must not compromise pot life or working time.

4. Incorrect Application Technique

Bubbles may also be introduced during application, particularly when the coating is rolled too aggressively or repeatedly worked as it begins to cure.

Common application problems include:

Using an unsuitable roller or application tool.
Excessive rolling or overworking the coating.
Applying material at an incorrect film thickness.
Working beyond the recommended application time.
Applying epoxy over a poorly sealed substrate.
Using inappropriate application techniques for self-levelling systems.

Apply epoxy at the specified coverage rate and maintain a consistent wet edge. Avoid repeatedly rolling material once viscosity begins to increase.

For suitable self-levelling epoxy systems, a spiked roller may help release entrained air during the specified working period. It should not be used indiscriminately on all epoxy coatings.

5. Moisture in the Concrete

Moisture within concrete can contribute to blistering, adhesion failure and other coating defects. These problems may be confused with ordinary air bubbles or pinholes.

Moisture-related defects can occur where concrete has insufficiently dried, lacks an effective damp-proof membrane or is affected by rising damp or moisture vapour transmission.

Before coating, assess moisture conditions using appropriate testing methods and investigate any suspected water ingress.

Where significant moisture transmission is present, a specified system such as Fortis HydroFort VB may be required.

Ordinary epoxy primers should not be assumed to provide moisture vapour barrier performance.

6. Surface Contamination

Oil, silicone, grease, cleaning products and other contaminants can cause epoxy to pull away from parts of the surface.

This can produce circular craters or fish eyes that may initially be mistaken for pinholes.

Contamination should be identified and removed before coating. Mechanical preparation alone may spread some contaminants rather than eliminate them.

See Why Does Floor Coating Reject or Form Fish Eyes? for further troubleshooting advice.

How to Diagnose Bubbles and Pinholes in Epoxy Flooring

The location, appearance and timing of a defect can help identify the likely cause.

Observed Problem	Possible Cause	Recommended Action
Bubbles appearing on a warming concrete slab	Concrete outgassing	Control application timing and ensure adequate priming
Numerous small pinholes across porous concrete	Inadequate substrate sealing	Assess porosity and apply a suitable primer system
Fine bubbles distributed through the coating	Mixing or application-related air entrainment	Review mixing speed, equipment and rolling technique
Circular craters where epoxy pulls away	Silicone, oil or other contamination	Investigate and remove the contamination
Raised blisters appearing after curing	Moisture or adhesion-related failure	Investigate substrate moisture and coating adhesion

Where blistering or coating lifting is widespread, further investigation may be required before selecting a repair system.

See Why Does Epoxy Flooring Delaminate? for additional information on adhesion failure.

How to Repair Bubbles and Pinholes in Epoxy Floors

Minor bubbles and pinholes can often be repaired, provided the underlying coating remains sound and the cause of the defect has been corrected.

Repairing Minor Surface Defects

Allow the defective epoxy coating to cure sufficiently for mechanical preparation.
Inspect the surface and confirm that the coating is firmly bonded.
Sand or grind raised bubbles and defective areas as required.
Remove dust and loose material thoroughly.
Fill open pinholes or small voids using a compatible epoxy repair material where necessary.
Prepare the surface to meet the recoat requirements of the selected system.
Apply a compatible finish coat at the specified coverage rate.

Simply applying another thick coat over open pinholes may not eliminate the problem, particularly if the concrete continues to release air.

Repairing Widespread Defects

Where pinholes or bubbles affect large areas, more extensive preparation may be necessary.

The defective coating may need to be mechanically removed to sound material before the concrete is re-primed and recoated.

If moisture-related blistering or adhesion failure is suspected, investigate the substrate before installing another coating system.

How to Prevent Bubbles and Pinholes in Epoxy Flooring

Successful prevention depends on controlling the substrate, materials and application conditions.

Mechanically prepare the concrete to the specified surface profile.
Remove dust, contamination and weak surface material.
Check substrate moisture and temperature conditions.
Apply epoxy while the substrate temperature is stable or falling where practicable.
Seal porous concrete with a compatible primer.
Use suitable low-speed mixing equipment.
Avoid introducing excessive air during mixing and application.
Apply coatings at the specified coverage and film thickness.
Follow the manufacturer's working time and recoat requirements.
Allow each coating layer to cure under suitable conditions.

For information on complete coating systems, see How Many Coats Does an Epoxy Floor Need?.

Fortis Epoxy Flooring Products

Fortis Adhesives & Coatings manufactures epoxy primers and floor coatings in Melbourne, Australia, for industrial, commercial and residential applications.

Fortis 823 Prime — fast-curing epoxy primer for properly prepared porous concrete.
Fortis 820 Prime — high-adhesion primer for poor, porous or difficult concrete.
Fortis Coat 823 HS — high-solids epoxy floor coating for compatible industrial and commercial flooring systems.
Fortis AquaCoat — low-odour water-based epoxy coating that can self-prime suitable concrete.
Fortis HydroFort VB — moisture vapour barrier for suitable specified applications.

Explore the complete Fortis Epoxy Floor Coatings range for product information and technical specifications.

Frequently Asked Questions

Why does epoxy flooring bubble during application?

Epoxy can bubble when air escapes from porous concrete or becomes trapped during mixing and rolling. Applying epoxy to a warming slab can increase the risk of concrete outgassing.

What causes pinholes in epoxy floor coatings?

Pinholes commonly form when trapped air escapes through wet epoxy or bubbles burst before the coating cures. Porous or inadequately sealed concrete is a common contributing factor.

Will epoxy pinholes disappear as the coating cures?

Not necessarily. Once the coating begins to cure, open pinholes may remain visible and may require filling, surface preparation and recoating.

Can I apply another coat of epoxy over pinholes?

Yes, where the existing coating is sound and properly prepared. However, open pores may need filling or additional priming. Recoating without correcting the cause can result in recurring defects.

Does epoxy primer prevent bubbles?

A suitable epoxy primer can reduce air release from porous concrete by sealing surface pores. It cannot eliminate all bubbles caused by mixing, application technique, moisture or unsuitable conditions.

Should epoxy be applied when concrete is warming or cooling?

Applying epoxy while the concrete temperature is stable or gradually falling generally reduces the risk of outgassing. Always remain within the manufacturer's application temperature and dew-point requirements.

Are epoxy bubbles caused by moisture?

Some bubbles and blisters are moisture-related, but many are caused by trapped air or outgassing. Moisture testing and inspection may be needed to determine the cause.

Can bubbles in cured epoxy flooring be repaired?

Yes. Minor defects can often be sanded or ground, filled where required and recoated using compatible materials. Widespread blistering or adhesion failure may require more extensive removal and substrate investigation.

Related Fortis Technical Resources

Need Technical Assistance?

Preventing epoxy bubbles and pinholes requires appropriate concrete preparation, primer selection and application conditions.

Fortis Adhesives & Coatings manufactures epoxy flooring products in Australia and can assist with selecting compatible primers and coating systems.

For technical advice on epoxy flooring defects or selecting a suitable coating system, contact the Fortis technical team.

Technical note: This guide provides general troubleshooting information. Actual coating defects may require on-site inspection, adhesion testing or moisture assessment. Always refer to current Fortis Technical Data Sheets and project specifications before application.