Uneven epoxy floors: causes

An uneven epoxy floor is a surface that is not uniformly flat: high and low spots, ripples, roller marks or trowel marks that are clearly visible when viewed at an angle under light. The floor may still be hard and well bonded, but an uneven surface causes real everyday problems: trolleys and forklifts jolt as they pass, water ponds in low spots, dirt is hard to clean out of ripples, and aesthetically the floor looks unfinished even though the coating has fully cured.

Illustration: a factory concrete floor seen from above — the flatness of the original substrate largely determines the flatness of the coating on top. Photo: Tiger Lily on Pexels, not a Kadier project.

Uneven because of the substrate or the coating?

The first question when facing an uneven floor is which layer the defect sits in. Epoxy coatings, especially thin roller systems, tend to “copy” the shape of the substrate beneath: a rough substrate gives a rough coating, only slightly smoother. Conversely, if the substrate was flat but the coating still ripples, the cause lies in the coating step — uneven thickness, uneven rolling or pouring, or a coating that did not flow flat before curing.

A simple check is to lay a long straightedge along the floor or stretch a string along it and measure the gaps at different points, combined with raking light to reveal ripples. If the ripples line up with the substrate’s own bumps (expansion joints, old patches, unevenly poured concrete), the cause is the substrate. If the ripples run along the rolling or pouring direction, the cause is the coating. This matters because fixing the wrong layer will not remove the unevenness: adding more paint over a rough substrate only makes the coating thicker while the surface stays rough.

Causes from the concrete substrate

Substrate poured unevenly and not levelled before coating

Cast-in-place concrete is rarely perfectly flat. Level differences, hollows from uneven compaction, and old patches at different heights all become unevenness in the epoxy floor if not dealt with first. A thin roller system has almost no levelling ability: a coat only a few hundred micrometres thick cannot fill height differences measured in millimetres. Even a self-levelling system has its limits — see the description of LevelFlow — self-levelling system: it covers substrate defects well but still needs the substrate prepared to the right standard; pouring thicker does not automatically mean flatter.

Expansion joints, cracks and old patches

Expansion joints that have settled unevenly on either side, filled cracks standing proud or sunk, and cement patches that shrink differently from the surrounding floor all form ridges or channels running across the coated floor. These points also move over time, so the coating above can crack along with them if the joints are not treated correctly. Hairline cracking and its remedies are covered separately in the article on epoxy floor crazing.

Causes from coating application

Uneven thickness

With a roller system, pressing hard at the start of a pass and lightly at the end produces coats of different thickness, visible as bands when viewed at an angle. With a self-levelling system, uneven pouring, inconsistent notched-trowel work or late defoaming also leave thick and thin spots. The amount of paint for each area should be calculated in advance from the area, rather than poured by eye and spread out.

Coating not flowing flat before curing

Self-levelling paint needs time to flow into a flat surface. High temperatures shorten this time, and the paint skins before it has fully levelled. Working a large area without dividing it into bays or joining batches in time also leaves ridges at the joints between two batches. Draughts and direct sunlight make the surface skin unevenly, making ripples more obvious.

Dust, contamination and roller marks

Dust falling into wet paint, grit mixed into the paint, or a dry, fibrous roller leaves ripples and raised particles on the surface. Contamination during application is covered separately in the article on epoxy coating contamination and dust. Roller marks left after the paint dries are usually caused by re-rolling once the paint has started to set, or by using a roller unsuited to the paint’s viscosity.

How does unevenness affect use?

Not every uneven floor needs immediate repair. In a low-traffic store, slight ripples affect only appearance. But on forklift routes and trolley areas, low spots pond water, making wheels slip, standing water soils and turns slippery over time; ridges on walkways jolt vehicles and can shift loads. In areas with high hygiene requirements, such as food or pharmaceutical plants, low spots that hold wash water are hard to clean and hard to get accepted at handover. So the need for repair should be judged by each area’s function, not for the floor as a whole.

Fixes by severity

Mild — surface ripples, roller marks, small height differences, no effect on traffic: the whole surface can be sanded to flatten the ripples, cleaned, and recoated with a thin coat. This only fixes shallow ripples, not large height differences caused by the substrate.

Moderate — localised hollows, ridges at joints, moderately rough substrate: high spots need grinding flat, low spots need filling with a material compatible with the coating system, then recoating. The filler must bond well to both the substrate and the old coat, or it will peel after a time and the unevenness will return at exactly that spot.

Severe — large height differences in the substrate, settled joints, widespread unevenness: the whole substrate needs reassessment, possibly wide-area grinding or a self-levelling levelling coat before the finish coat. This is the most expensive option and takes the floor out of use the longest, so it should only be done after measuring and agreeing the flatness to be achieved — absolute flatness for an entire factory is a very different requirement from flatness sufficient for smooth forklift travel, and the costs differ greatly.

Frequently asked questions

Is an uneven epoxy floor caused by poor paint?

Usually not. In most cases the cause is an unlevelled substrate or uneven application. Changing the paint while keeping the same rough substrate and the same application method will still give an uneven new floor.

Will another self-levelling coat flatten an old floor?

Yes, but within limits. A self-levelling coat levels better than a roller system, but if the height differences are too large, it will flow into the hollows and stay thin on the high spots, and may even need several coats to reach the desired flatness. Before coating, measure the actual height differences to work out the paint quantity and number of coats needed, rather than expecting a single coat to flatten everything.

How should epoxy floor flatness be accepted at handover?

Agree the measurement method with the contractor in advance: place a long straightedge on the floor and measure the largest gap beneath it at many points, or use raking light to check ripples. Agreeing after the work is finished is very difficult, because each side understands “flat” to a different standard. Substrate preparation and flatness requirements should be in the construction documents from the start — see also the article on concrete surface profile (CSP).

A note on specifications

Official technical data (TDS) for each Kadier product is published on the Products page; this article sets out a general framework and does not replace the TDS of a specific product. Standards such as TCVN 9014:2011 and TCVN 12574-2018 set the general framework for epoxy paints and coatings; in actual application, self-levelling coat thickness, bay division and joint timing must follow the TDS of the specific product to be used. Project owners should ask the supplier to provide the TDS and agree in writing the flatness to be achieved for each area before application.

If your floor is uneven, ponds water in low spots, or makes forklifts jolt as they pass, send photos and the locations of the uneven areas for advice on sanding, patch filling or self-levelling top-up — see Get a quote and contact Kadier for an on-site survey.

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