Epoxy floor delamination: causes

Epoxy floor delamination is when coats of paint separate from each other: the top coat peels off in patches, exposing the intact coat below. The key sign is that the underside of the peeled patch is still smooth and carries the colour of the coat beneath, while the concrete substrate is not exposed. Delamination differs from peeling: peeling is the whole coating system separating from the concrete substrate, usually because of a weak or damp substrate — covered separately in the article on epoxy floor peeling from a weak substrate. Delamination is a bonding failure between coats, and its fix is different: there is no need to treat the substrate in depth; the interface between the two coats needs to be reworked.

Illustration: coating peeling off in patches, exposing the coat beneath — when a coat cannot bond to the coat below, delamination shows clearly patch by patch. Photo: Peter Holmboe on Pexels, not a Kadier project.

Why don’t two coats bond to each other?

Epoxy bonds to an old epoxy coat by two mechanisms: chemical bonding while the lower coat has not fully cured, and mechanical bonding when the lower coat’s surface is sanded to create grip. Delamination happens when both mechanisms are weak: the lower coat has cured for too long without being sanded, or the interface surface is contaminated. In other words, the top coat merely “lies” on the lower coat without truly bonding to it, so any trigger — traffic, temperature change, water seeping into an edge — peels it off.

Common causes

Recoating too late, past the recoat window

Every epoxy product has a permitted time window for applying the next coat, called the recoat window. Coating within this window gives good chemical bonding between coats. Coating after the window has closed, when the lower coat has cured hard with a glossy, smooth surface, leaves no chemical bond. This is the most common cause of delamination, and it usually happens when the work schedule is interrupted: a weekend break, waiting for materials, or bad weather pushing the next coat back by several days.

Lower coat not sanded when needed

Once the recoat window has passed, the correct process is to sand the entire surface of the lower coat to create mechanical grip, then clean it thoroughly before applying the top coat. Skipping the sanding step — to save time, or assuming a still-new surface will still bond — directly causes delamination. Similarly, patchy sanding, with some areas done and others missed, makes delamination appear in blotches: sanded areas bond, unsanded areas peel.

Contaminated interface surface

Grinding dust, grease, water, condensed moisture or spilled chemicals on the lower coat all become a separating layer between the two coats. Especially risky is a lower coat left overnight in a factory in operation: production dust and oil vapour from machinery can settle on the surface invisibly to the naked eye. Applying the top coat over a dirty surface gives weak bonding even if the recoat window is still open. Cleaning and inspecting the surface before the next coat is therefore a mandatory step, not an optional one.

Incompatible coats

Using a primer from one product and a topcoat from another, or applying a water-based coat over an oil-based coat (and vice versa) without confirmed compatibility, can cause delamination even when timing is right. Each coating system is designed so its coats bond to each other; mixing systems creates risk on its own. With a multi-coat roller system such as RollCoat — 3-coat roller system, one primer and two topcoats must be treated as a single unified system, not swapped coat by coat without the supplier’s compatibility confirmation.

Applied under unsuitable conditions

Applying the top coat while the lower coat is still wet from insufficient drying time, or while the surface carries condensed moisture, also causes delamination — by a different mechanism: the top coat does not bond to a damp surface and shrinks away as it dries. Drying time between coats and dew-point conditions are covered in the articles on epoxy floor drying times and dew point in epoxy application. A wrong mix ratio also leaves a coat incompletely cured with a weak surface that the next coat struggles to bond to — see the article on epoxy mixing ratio and pot life.

Diagnosis: is it really delamination?

Tap lightly on the suspect area: a delaminated area sounds hollow and empty compared with a well-bonded area. Pry up a peeling edge: if the underside of the peeled patch is the old coat intact and the concrete is not exposed, it is delamination. If concrete is exposed with cement dust, it is peeling from the substrate — a very different fix requiring substrate and moisture checks. If the peeled patch crumbles and comes with round blisters, compare with subfloor moisture blistering, since moisture can both blister and weaken bonding.

Also determine how far the delamination extends: tap outward around the visibly peeled area, because the top coat may already have separated from the coat below over a wide area without peeling off yet. Repairing only the visibly peeled area and missing the separated-but-not-yet-peeled area means that area will peel later.

How to fix it

The first step is always to remove the entire delaminated coat, including areas that have separated but not yet peeled, down to a firmly bonded surface. Keeping part of a separated coat in the hope that a new coat will “glue” it down is a mistake: a new coat cannot restore the lost bond between the two old coats.

After removal, sand the remaining coat’s surface, clean off dust and check for contamination (grease, moisture). If the surface is still greasy, degrease it and let it dry completely before coating. Then reapply the topcoat following the coating system’s process, within the new coat’s recoat window if several coats are needed. For large delaminated areas or delamination that returns after repair, review the whole original application process: the coat-application log, interruption times, sanding and cleaning steps — because repairing without fixing the process means delamination will recur in the next area. The process for patching and recoating local areas is covered separately in the article on localized epoxy floor repair.

Frequently asked questions

Can delamination be repaired locally?

Yes, if the delaminated area is small and the boundary of the separated area has been clearly identified. Local repair must remove all separated material, sand, clean and recoat, accepting that the joint edge between the repaired area and the old floor may be visible. For large areas, recoating the whole area usually gives a more even and durable surface than patching spot by spot.

Will a new coat over a delaminated coat hold the old coat down?

No. The new coat only bonds to the already-separated coat, while that coat remains separated from the coat beneath it. The whole stack of upper coats will still peel off together under stress, only a little later.

How can delamination be prevented from the start?

Three key points: apply coats within the product’s recoat window; if the window has passed, sand thoroughly and clean before recoating; and do not mix coats from different coating systems without confirmed compatibility. An application log recording the time each coat was applied is important evidence for identifying the cause if delamination occurs later.

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, recoat windows, sanding requirements and surface conditions must follow the TDS of the specific product to be used. Project owners should ask the supplier to provide the TDS and keep a log of each coat’s application time for comparison if delamination occurs.

If your floor is peeling in patches and exposing the old coat beneath, send photos of the underside of the peeled patches and information about when the coats were applied, so the problem can be correctly diagnosed as delamination or peeling — see Get a quote and contact Kadier for an on-site survey.

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