Epoxy Coating That Won't Dry: Causes and Fixes

An epoxy coating that remains tacky after application, with a soft surface, marks left by a light touch, or easy peeling under foot traffic, is a sign of a failure to dry or slow drying. This article helps you diagnose the correct group of causes before taking action, rather than applying another coat over a surface that has not yet cured.

This article focuses only on diagnosing the failure to dry. The overall application process is given in the article Standard Epoxy Flooring Installation Process, from Survey to Handover; mixing methods and the pot life of the mixture belong to the article Epoxy Mixing Ratio and Pot Life of the Mixture; the drying time frame and return-to-service time belong to the article Epoxy Floor Drying Times and Return-to-Service Times; and how to measure dew point during application belongs to the article Dew Point in Epoxy Application: Why Measure It?.

What does “not dry” mean?

Not every surface that has not yet hardened is the same failure. The following states need to be distinguished:

Surface still tacky: touching it feels sticky, dust adheres easily, fingerprints show clearly. This is the most common form and also the easiest to confuse with slow drying caused by environmental conditions.
Soft coating, leaving indentations: light pressure leaves an indentation, or a fingernail leaves a mark. The coating has not yet formed a firm film and cannot yet withstand foot traffic or heavy objects.
Dry on the surface but soft inside: the outer surface appears to have set, but the layer beneath is still soft and easily blisters or peels under load. This form is dangerous because it is easily misjudged as satisfactory.
Localised failure to cure: only one area is tacky or soft, while other areas have already hardened. A localised form usually suggests the cause lies in that area — for example, localised substrate moisture or uneven mixing in one batch of paint.

Recording the correct state and scope (the whole floor or localised, which coat is affected, which areas are affected) is the first and most important step, because each form leads to a different remedial direction.

Quick checks before concluding

Before attributing a cause, check in the following order and record the results:

Compare areas: is the whole floor affected, or only a few spots? If only a few spots, delineate them and compare with the substrate condition in exactly those areas.
Identify the affected coat: primer, intermediate coat, or topcoat? Which coat has not dried will decide the scope to be treated, rather than treating the whole coating system.
Recheck the application log: which paint batch, who mixed it, mixed to which instructions, applied under what weather conditions, and whether the coat was applied thicker or thinner than other coats. These records usually point to the cause faster than any guesswork.
Check the substrate on site: whether the substrate shows signs of moisture, standing water, or rising damp, especially in the affected area. The article on damp substrates and moisture-barrier solutions will be useful when this group of causes is suspected.
Do not conclude hastily before enough time has passed: a coat may only be drying slowly due to environmental conditions and may not yet be damaged. Compare with the time frame in the technical documentation of the specific product before deciding to intervene.

Cause group 1: Incorrect or uneven mixing

Epoxy paint is a two-component system of resin and hardener, and it cures correctly only when the two components are mixed and stirred as the manufacturer directs. This group of causes usually produces failure to cure across the whole area of one paint batch, or patchily if mixing was uneven.

Suggestive signs: a uniformly tacky surface across the area of one application batch, while other batches have hardened; or a surface with hard and soft spots alternating within the same area.

How to check: compare the affected batch with batches that have dried well; review the mixing procedure for that batch, including measuring the components, stirring time, and stirring tools; check whether any batch was mixed with the wrong components or the wrong accompanying hardener.

Remedial approach at the level of principle: a coat that has not cured because of incorrect mixing usually cannot be saved by waiting longer, because the curing reaction lacked the right conditions from the start. The approach in principle is to assess the extent, remove the uncured portion as directed by the contractor or supplier, and then recoat following the correct mixing procedure. Never overcoat an uncured coat in the hope that the new coat will dry the old one.

Cause group 2: Unsuitable temperature and humidity during application

The curing reaction of epoxy paint is directly affected by the temperature and humidity of the application environment. Low temperature slows the reaction, high humidity makes the surface vulnerable while the paint film is still wet, and a difference between the substrate surface temperature and the dew point can cause moisture condensation on the surface being coated.

Suggestive signs: the whole floor drying slowly and uniformly after a cold or humid weather spell; the surface showing dullness, slight pinholing, or uneven gloss together with slow drying.

How to check: compare the actual application conditions with the permitted conditions stated in the product’s technical documentation; check substrate temperature, air humidity, and dew point at the time of application if recorded data exist; review ventilation and protection of the application area.

Remedial approach at the level of principle: if the coat is only drying slowly due to environmental conditions while the mix remains correct, the usual approach is to improve the conditions in the application area and wait the full time required by the product documentation, while protecting the surface from dust, water, and foot traffic during the wait. Where the surface has already been clearly affected, such as dullness, pinholing, or soiling, reassess before deciding whether to treat the surface or recoat.

Cause group 3: Coat applied too thickly

Each coating system is designed with a thickness for each coat. Applying one coat thicker than designed, or piling several wet coats on top of each other when the lower coat has not yet reached the state permitting the next coat, can make the coating slow to dry, dry on the surface but soft inside, or develop air bubbles and blistering.

Suggestive signs: places where paint has pooled thickly, paint puddles, or wall bases where paint easily accumulates, drying much more slowly than other places; a surface dry on top but soft when pressed beneath.

How to check: compare the actual thickness at the affected location with the designed thickness of the coating system; check whether the lower coat had reached the condition for receiving the next coat before the upper coat was applied; review the application tools and workmanship in the thickly pooled area.

Remedial approach at the level of principle: do not add more coating to fill the thick spots, because additional coating only worsens the slow drying. Wait for assessment, or treat the excessively thick portion as advised by the contractor, and only then continue application at the correct designed thickness.

Cause group 4: Damp substrate or rising moisture

A concrete substrate that is still damp, moisture rising from beneath the substrate, or water seeping through cracks and construction joints can all prevent the coating from adhering and curing normally. This group of causes is often overlooked because the substrate surface may look dry on the outside while the inside is still damp.

Suggestive signs: the failure concentrated on ground floors, in basements, near water sources, cracks, or places that previously showed signs of ingress; accompanied by blistering, peeling, or damp marks after the coating has been applied.

How to check: measure substrate moisture in the affected area and in a control area that has dried well; check ingress marks, construction joints, pipe collars, and wall bases; compare with pre-application substrate survey results to see whether this area was ever recorded as a damp substrate.

Remedial approach at the level of principle: treating substrate moisture is a prerequisite before treating the coating, because recoating on a still-damp substrate will repeat the failure. Depending on the moisture level, the option may be to wait for the substrate to dry, to treat moisture protection, or to use a coating system solution suited to damp substrates as advised by specialists. Which option to choose must rest on actual moisture measurement results; it should not be decided by visual observation alone.

Things not to do when the coating has not yet dried

Do not walk on, drag heavy objects over, or place equipment on an undried surface to “test whether it has hardened”, because this leaves marks and damages the film that is forming.
Do not apply another coat over an uncured coat, and do not add solvents or thinners on your own in the hope of making the paint dry faster.
Do not apply a direct heat source to one point at high intensity, because uneven heat can deform the surface while the layer beneath has still not cured.
Do not clean with strong cleaning chemicals while the coating has not yet hardened, because the film at this stage is very sensitive and easily corroded on the surface.
Do not cover the surface tightly with tarpaulin or non-breathable material before the cause has been clearly established, because this can trap moisture and prolong slow drying.

When to wait longer, and when retreatment is required

The distinguishing principle is as follows.

Wait longer when the mix is correct, the substrate meets requirements, and the coat is only drying slowly due to environmental conditions that can still be improved; during the wait, protect the surface and recheck periodically rather than leaving it unattended.

Retreat when it has been established that the coat has not cured because of incorrect mixing, untreated substrate moisture, or an excessively thick coat accompanied by surface deformation; in these cases, waiting longer usually does not make the coat improve on its own.

The decision to retreat must rest on an on-site assessment by the contractor or supplier, covering the affected scope, which coat is affected, and the cause already identified. For projects requiring handover acceptance, every remedial option should be recorded in writing as the basis for re-acceptance after rectification.

Prevention for the next application

Survey and measure substrate moisture before application, recording separately any areas suspected of dampness so that treatment can be arranged beforehand, rather than discovering it after coating.
Mix exactly as the manufacturer directs for each batch, stir evenly, and record paint batches so that, when a failure occurs, the correct batch and area can be traced.
Monitor and record temperature, humidity, and dew point throughout application, especially when the weather changes during the day.
Apply the designed thickness for each coat, without piling coats on when the lower coat has not yet reached the condition for receiving the next coat.

Whether it is a roller-applied system such as RollCoat or a self-levelling system such as LevelFlow, the diagnostic principle for a failure to dry remains the same: identify the correct cause group first, and only then choose a remedial option.

Note on specifications

Kadier had not published an official technical data sheet (TDS) for its coating systems, so this article presents only a general diagnostic framework and remedial approaches at the level of principle, and gives no specific Kadier values for mixing ratio, drying time, or temperature and humidity thresholds. Every specific value applied to a project must follow the TDS of the specific product used at that project, together with the contractor’s and supplier’s instructions. The general reference framework in this article is consistent with the approach of coating standards such as TCVN 9014:2011 and TCVN 12574-2018, but these standards do not replace the product TDS for any specific case. Where there is a conflict between the general framework in this article and the product TDS, the product TDS is the applicable basis.

If your epoxy floor is not drying and the cause group has not yet been established, please contact us for a survey and advice on a suitable remedial plan: Contact Kadier.

Illustration: hands using a paint roller with a paint bucket and brushes on a painting project. Photo: Ivan Samkov / Pexels. See the original photo at Pexels.

Uncategorized

Epoxy Coating That Won’t Dry: Causes and Fixes