Why Do Epoxy Floors Blister? Causes and Prevention

Short answer: What people call "blistering" in an epoxy floor has five different causes: moisture from the concrete, air from porous concrete or from mixing, loss of adhesion linked to surface preparation, mixing problems, and condensation or temperatures outside the data sheet range. Moisture is the most common cause; DEMCO data sheets require substrate moisture below 4% and ambient humidity below 80% for epoxy application. When and where a blister appeared, and what is found underneath it, point to the cause, and the repair is chosen accordingly.

The limits and values below are taken from the technical data sheets of DEMCO primers and floor coatings.

Five different defects called "blistering"

Every bubble, pinhole or lifted patch in a coating tends to be called a blister, yet these defects appear at different times and are prevented in different ways. Moisture testing and primer selection are covered in the Concrete moisture before epoxy guide.

1. Moisture from the concrete

Once the coating seals the surface, water vapor in the concrete can no longer escape; as the concrete warms up, the vapor pressure rises and pushes the coating up where adhesion is weakest. If dissolved salts draw water under the coating (osmosis), the blister fills with liquid. These blisters appear days or weeks later and can grow; they are most common on new concrete, on slabs affected by rising damp, at column bases and along external walls.

According to the data sheets of DEMCO EPOX 100 PR and DEMCO EPOX 2000 SELF, substrate moisture must be below 4% and ambient humidity below 80%, and the coating must not come into contact with water for 24 hours after application. On sound but damp concrete, surface-tolerant moisture barrier primers (DEMCO EPOX 110 PR, DEMCO EPOX 112 PR, DEMCO EPOX 114 PR – FAST) are used; their data sheets give no numerical moisture limit. The DEMCO EPOX 192 NOVOLAC TOPCOAT data sheet states that the product must not be used on substrates with rising damp.

2. Air and gas bubbles, pinholes

The air in the pores of the concrete expands as the floor warms up and rises through the still-liquid coating. If the coating is still fluid, the bubble bursts and the mark levels out; if it has started to thicken, a pinhole or small crater remains. Air drawn in during mixing produces the same appearance. These bubbles form during application and hardening, and do not grow afterwards.

According to the data sheets, the concrete is primed with DEMCO EPOX 100 PR so that it is fully impregnated; the primer penetrates the capillary pores of the concrete and blocks them. In a self-leveling coating (DEMCO EPOX 2000 SELF), the air bubbles that form are removed with a spiked roller. The DEMCO EPOX 2004 SELF/FAST data sheet asks for spike rolling within about 10 minutes of application where possible, and warns against over-rolling. For details, see Self-leveling or orange-peel textured floor coating?

3. Loss of adhesion (delamination)

Here there is no pressure under the coating; the coating has not bonded well enough to the surface. Typical signs are areas that sound hollow when tapped and coating that lifts off in sheets from an edge or a crack; the underside of the lifted piece is usually clean, dusty, oily or glossy.

The data sheets require the surface to be free of oil, rust and dust, with no loose layer left, and glossy surfaces to be roughened by shot blasting, grinding or scarifying (DEMCO EPOX 110 PR and 112 PR). Adhesion strength (pull-off, TS EN 1542) is given as above 2 N/mm² in the DEMCO EPOX 100 PR data sheet and at least 1.5 N/mm² in the DEMCO EPOX 2000 SELF data sheet. The DEMCO EPOX 158 TOPCOAT data sheet gives the value as "> 2 (failure in concrete)": in the test, the concrete broke, not the coating. In other words, the bond is only as strong as the surface beneath it; if a loose or contaminated layer remains, the coating lifts off together with it.

4. Mixing ratio and mixing

A two-component epoxy only cures when it is mixed in the correct ratio and thoroughly. If the ratio is off, or unmixed material remains at the bottom of the container, soft, sticky patches form, and these areas may lift over time. Mixing at high speed or for longer than necessary draws air into the mix.

The data sheets give the ratio by weight: for DEMCO EPOX 2000 SELF, the A/B ratio is 80/20, and the 20 kg set contains 16 kg of component A and 4 kg of component B. All of component B is poured into component A and mixed for at least 3 minutes, at low speed to prevent air bubbles. The DEMCO EPOX 192 NOVOLAC TOPCOAT data sheet also states that mixing longer than necessary will create air bubbles. These products are not thinned with thinner.

5. Condensation, dew point and temperature

When the surface temperature approaches the dew point, moisture in the air forms a thin film of water on the surface that may not be visible. A coating applied over it does not bond well; the surface may turn dull, whitish or remain sticky. Temperature also affects bubbles: in the cold the material thickens and air cannot escape to the surface; on a floor that warms up after application, the air in the pores expands.

According to the DEMCO EPOX 100 PR and 2000 SELF data sheets, the working area and substrate temperature should be 15 – 30 °C, and the substrate must never fall below 10 °C. The DEMCO EPOX 158 TOPCOAT data sheet requires the surface temperature to be at least 3 °C above the dew point to prevent condensation; the DEMCO PUR 303 CONCRETE SL 23 UT/WB data sheet requires this for 48 hours after application as well. DEMCO EPOX 192 NOVOLAC TOPCOAT must be protected from moisture, condensation and water for at least 24 hours after application.

Symptom, likely cause and prevention

Symptom	Likely cause	Prevention (per DEMCO data sheet)
Growing blister that appears days or weeks later; damp underneath or filled with liquid	Moisture from the concrete or rising damp, new concrete that has not dried	Substrate moisture below 4%, ambient humidity below 80%; on sound but damp concrete, DEMCO EPOX 110 PR, 112 PR or 114 PR – FAST
Bubbles during application that leave pinholes or craters	Air escaping from porous concrete	Impregnating primer DEMCO EPOX 100 PR; epoxy putty for cracks and holes; spiked roller on self-leveling coatings
Fine bubbles spread through the film	Mixing at high speed or for too long	Mixing at low speed for at least 3 minutes
Coating that sounds hollow when tapped or lifts off in sheets	Oil, dust, a loose layer or a glossy surface	Removing contaminants and loose layers; roughening by shot blasting, grinding or scarifying
Soft, sticky or discolored patches	Incorrect ratio, incomplete mixing, added thinner	Keeping to the ratio by weight, pouring all of component B into A; no thinning
Dull, whitish or sticky surface	Condensation, high ambient humidity	Ambient humidity below 80%; surface temperature at least 3 °C above the dew point
Bubbles, slow or incomplete curing	Temperature outside the data sheet range, floor warming up after application	Working area and substrate 15 – 30 °C; substrate never below 10 °C

How to tell the cause on site

• When did it appear? A bubble that forms during application is usually air; a blister that appears days or weeks later and grows points to moisture.
• Where are they concentrated? Blisters on slabs on ground, at column bases and along external walls are linked to moisture; pinholes spread across the area to porous concrete; patches in strips to mixing.
• What is underneath? If the cut blister is wet underneath or filled with liquid, the cause is moisture; if it is dry and cratered, it is air. If the underside of a lifted piece is clean, dusty or oily, the problem is surface preparation; if a thin layer of concrete comes off with it, the surface layer of the concrete is weak.

What to do if blistering has already occurred

Find the cause first; otherwise the repaired area may blister again. Piercing a blister and filling it does not remove the source of the moisture.

1. The blistered and hollow-sounding area is cut out back to sound coating.
2. Because the cause is usually moisture, the moisture of the exposed concrete is measured and its source investigated.
3. The area is prepared again and re-primed: with an impregnating primer on dry concrete, with a moisture barrier primer on sound but damp concrete.
4. The coating is applied again under the temperature and humidity conditions in the data sheet.

If blistering is widespread, keeps growing or the moisture comes from below, the repair method should be assessed together with our technical team.

Frequently asked questions

Do bubbles in an epoxy floor go away on their own?

No. Pinholes and craters in a cured coating are permanent. Moisture blisters may seem to flatten when the floor dries, but the coating has separated from the concrete in that area and can blister again as long as the moisture persists.

How is a blistered epoxy floor repaired?

The blistered area is cut out, the concrete is checked for moisture, the surface is prepared again and primed, and the coating is reapplied. On sound but damp concrete, a moisture barrier primer is chosen; for large areas, the technical team decides on the method.

How long after coating does blistering appear?

It depends on the cause. Air bubbles form during application and hardening and do not grow afterwards. Moisture blisters can appear days, weeks or even months later and can grow over time.

Does a spiked roller prevent bubbles completely?

No. A spiked roller removes the air that gets into a self-leveling coating during mixing and application; it does not solve air or moisture that comes out of the concrete later. Those risks are reduced by an impregnating primer, moisture testing and the temperature range in the data sheet.

Tell us when and where the blistering appeared, the age of the floor and any readings you have in the quote form, and our technical team will assess the cause and a suitable system. For the application steps, see How to apply an epoxy floor coating, and browse our floor coating products.