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Understanding the different epoxy bonding strengths, curing times and the importance of substrate preparation and integrity are vital components for ensuring the reliability of any epoxy flooring system.

In this special feature, we discuss the top reasons for failure, how to avoid them in future and finally, how to rectify the failure once it has occurred.

Here’s a look into the top nine reasons why epoxy flooring systems go wrong.


A weak substrate will always result in product failure. All epoxy resin products have very high bond strengths when fully cured. The bond strength of a standard epoxy system is >1.5MPa which means that the concrete needs to have a cohesive tensile strength of no less than 1.5MPa to cope with the epoxy.

If the epoxy bond strength is higher than the concrete cohesive strength, the epoxy will begin to crack and delaminate, pulling parts of the concrete with it, hence the concrete will break away from itself.

It is also essential for the following notes, Sharon Margon, Technical Advice Supervisor at TAL.

• The substrate must be fully cured and have attained the moisture content stipulated by the manufacturer of the floor covering. Excessive moisture in the substrate, or high moisture vapour emission rates, will most certainly result in an installation failure, for example, vinyl flooring ‘bubbling’ or delaminating from the substrate.

• The substrate must be integrally sound, smooth and level. Vinyl, laminates, resin coatings and large format tiles will suffer from surface imperfections or a poor aesthetic appearance if the floor below is not perfectly smooth and level.

If the epoxy bond strength is higher than the concrete cohesive tensile strength, the epoxy will delaminate due to surface tension and insufficient tensile strength > 1,5MPa, pulling parts of the concrete with it, hence the concrete will have cohesive failure.

Mark Griesel, Sika’s regional manager notes, “A minimum substrate compressive strength of 25MPa is recommended for almost all epoxy resin systems. In South Africa, poor concrete has become the status quo and sites need to be checked properly before starting. The following test must be conducted after surface preparation – tensile adhesion test, Schmidt hammer test, substrate moisture measured with Tramex moisture meter.”


Whether applying a new coat of epoxy onto a new or an existing floor coating, it is most important that the surface is ready to accept the new coat or layer. The surface needs to be clean, dry and free of debris.

TAL Screedmaster

The best way to prepare the substrate is to either vacuum shot-blast or diamond grind. Surface preparation ensures that the surface laitance has been removed, open texture surface is achieved and a desired Concrete Surface Profile (CSP) is achieved that will allow the epoxy resin to properly adhere to the surface. CSP 2 and 3 is recommended for epoxy resin applications.

Diamond grinding is the most popular method, especially when used with an attached vacuum to remove the dust and loose debris. Most contractors use diamond grinding as a vacuum shot blast machine is very expensive.

Sharon Margon, Technical Advice Supervisor at TAL comments that the installation of a suitable levelling compound is key to ensuring a sound, smooth and level substrate prior to installation of the final floor covering and includes the following.

• For an epoxy resin floor coating, to successfully bond onto the substrate, it must be free of all contamination, and must be suitably prepared. When installing over degraded surfaces which are contaminated with oil, grease etc, the surface may need to be chemically stripped or degreased, followed by mechanical preparation abrasion to render the substrate suitable for the installation of the flooring system.

• In cases where the substrate is inherently weaker than 30mpa, there could be delamination of the resin flooring system due to cohesive failure within the substrate itself. Unbonded or weak concrete should be removed and made good.

• Where no, or inadequate primer has been applied, excess moisture or liquids can build up on the underside of the epoxy flooring system. As this compounding effect is unable to evaporate, or pass through the epoxy resin, it causes blistering. A suitable and moisture tolerant primer is therefore an important part of a well-planned multi-level system.


For industrial grade epoxy floors, a primer must always be used. Some manufacturers don’t recommend a primer, but a primer coat is an integral part of the preparation. The primer will help seal and penetrate the pores of the prepared concrete and improve adhesion of the topcoat.

Primers generally have a lower viscosity that helps it infiltrate the substrate and bond the top layer to the floor from within and not only on the surface. It also prevents outgassing of the substrate through the topcoat.

A special primer is required when the need for damp tolerance or oil tolerance is specified.


Ensure the surface is free from rising damp/moisture and that the humidity is not higher than 75% RH. Epoxy resin systems generally do not work well in the presence of water. It is always advisable to survey a site first before beginning with any work because it is easy to see when moisture is around.

“Dew Points must be checked to ensure that condensation has not formed on the surface. Due point factors in: Substrate temperature, ambient temperature and relative humidity and the substrate temperature must be > 3K above due point using a thermometer, hygrometer and due point chart. An electronic due point meter is the easiest way to check due point,” says Sika’s Griesel.


Damp always darkens the substrate and has a distinct smell. The proper choice of primer is paramount to a successful epoxy floor installation as not all primer products are tolerant to moisture. A moisture meter is a vital tool to check moisture levels. Grinding a floor also goes a long way to assist with drying the substrate.


Epoxy resins undergo a chemical reaction when mixed, which means that mixing must be done correctly. The most skilled person in the team should do the mixing, as they will give due attention to mixing of the components correctly, batch sizes and ratios, comments TAL’s Technical Advice Supervisor, Sharon Margon.

It is important to always follow the manufacturer’s instructions. Working to a customised Methods and Materials specification for the project, also goes a long way to ensuring success.

A slow speed, high torque mixing tool is the best for proper mixing. A household drill is not an acceptable mixing device as it turns too fast and cannot cope with thick resins. If mixing is too vigorous, air might become trapped inside the substance, creating numerous potential complications during pouring and curing. Mitigate these issues by ensuring that your floor prep is adequate and the surface is clean of debris. Also, do not begin without all safety and protective measures in place.

Slowly mix epoxy for five minutes, scraping and the sides and bottom repeatedly, then pour and spread the mixture immediately.


Fish-eyes refer to round craters or beading in an epoxy layer and are typically the result of an epoxy that is spread too thinly over its surface or an indication air or surface contamination, which are the two reasons mostly causing fish eyes.

Fish-eyes are notoriously difficult to sand down once they have occurred but are preventable through due diligence. If epoxy is stressed (spread too thin) in a particular area before curing, you can add another layer of epoxy to mitigate the fish-eye.

However, the most efficient fish-eye “fix” is through proper floor (or surface) preparation and adhering to the manufacturers specified application thickness.


Proper epoxy installation yields a hard cure after 24-36 hours. If disproportionate amounts of part A and part B combine, it can compromise a complete cure. The mixture begins hardening immediately and needs to be poured straightaway to stall rapid hardening which makes the epoxy difficult to spread.

If you have any soft or sticky areas in your new epoxy surface, the mixing process most likely went wrong. In this case, your only option is to remove the original epoxy coat and start over with a new installation. Always mix the product as it is supplied in kit form.

Epoxy products are always a minimum of 2 parts (A + B) and sometimes there is a part C that needs to be added too. It is always better to waste what is not used than to have to buy additional material to redo the entire job. As a rule of thumb when quoting on work to be done, a small percentage (5-10%) for wastage is added.


Both the ground (concrete) and the ambient temperatures can negatively impact the epoxy’s chemical properties during curing and thus the installation. Standard epoxy has a chemically reactive threshold of 10°C, which is called the exothermic reaction.

Warmer temperatures accelerate while cold temperatures decelerate exothermic reactions. This reaction is when molecule cross-linking occurs. Before mixing, ensure that the epoxy is applied with a minimum substrate and ambient temperature of 10°C and maximum of 30°C.


A problem often occurs on a job site when the incorrect tools are used to apply the epoxy coating. An experienced flooring professional should supply the correct installation tools and equipment.

It is always best to find out from the supplier what the correct tools are to achieve the best and most desirable finish. For example, you cannot apply a 2mm self-levelling epoxy screed using a roller. The converse is also true. You cannot apply a thin coat of epoxy enamel with a notched rake. Systems are designed to be applied in a certain way for a reason.

Not using the correct tools can result in uneven distribution of the product, poor coverage, pin holes and bubbling of the epoxy flooring.

For more advice, read our special feature on resin and concrete flooring failures and how to avoid them.

Our sincere thanks and appreciation to the following manufacturers for sharing their expertise allowing us to prepare this article: , , and

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