Epoxy Primers Explained: Getting Subfloors Ready for Resin and Self-Smoothing Floors Why does the right primer make or break your floor?

Every experienced floor layer knows the truth: a resin or self-smoothing floor is only as good as what’s underneath it. You can spec the best resin floor on the market, mix it perfectly, and apply it with precision, but if the primer or scratch coat is wrong or skipped entirely, it can cause pinholes, delamination, debonding and a costly call back. The subfloor preparation stage determines everything that follows.

The epoxy primer is the critical anchor between the raw substrate and your finished system, whether that’s a self-smoothing compound, a comfort floor, a decorative resin coating, or an industrial topping. It seals the surface, regulates absorption, binds loose particles and provides the adhesive platform that subsequent coatings need to perform.

Arturo’s range of epoxy primers, damp-proof membranes (DPMs) and scratch coats have been engineered for professional installers. Whether you’re working on a sound power-floated slab, a moisture-affected screed or a rough substrate that needs to be smooth before your floor goes down, there’s a product in the range built for the job. Browse the full range here: Arturo Primers, DPMs and Scratch Coats.

The crosslinking chemistry is what sets 2-component epoxy primers apart from single-component or PU primers. Once Part A and Part B are combined and mixed, the reaction begins immediately, which is why on-site pot life management is essential. The resulting film is far denser and more chemically resistant than its single-component counterpart, making epoxy primer flooring application significantly more reliable in demanding environments. 

PU primers offer flexibility but are typically reserved for specific system pairings. For most concrete and cementitious screed substrates used with resin or self-smoothing systems, a 2-component epoxy primer is the professional’s choice.
 

Where Epoxy Primer Fits in the System

Epoxy floor primer sits at the base of virtually every professional resin system build-up. It’s used under: 

• Self-smoothing (Flow applied Floor) Compounds
• Comfort floors and decorative PU or EP Systems
• High Build Coatings 
• Resin Screed and broadcast system

Typical Substrates

An epoxy primer for flooring is suited to a wide range of substrates commonly encountered in resin floor layers.

• Power-floated or standard concrete slabs.
• Cementitious sand/cement and anhydrite screed
• Existing epoxy or resin surface (following adequate mechanical preparation)
• Dense or lightly contaminated surface where correct prep has been carried out

 

Key Checks Before You Select a Primer

Never commit to a primer until you’ve assessed the substrate. The checks that matter most are:

Moisture content and relative humidity (RH): Use a hygrometer box or a drill-and-plug.
Exceeding the threshold for standard epoxy primer means you need an epoxy DPM, using the wrong product here is one of the most common causes of floor failure.

Surface strength: Conduct a pull-off test or scratch test where required. Weak, friable, or dusty concrete must be mechanically prepared. No primer will compensate for a substrate that isn’t sound. 

Porosity and absorption: A simple water drop test tells you a lot. Highly porous substrates will require careful coverage management and may need a second coat.

Contamination: Oils, existing adhesive residues and curing compounds will all compromise adhesion. These must be removed mechanically before priming.

Surface defects: Cracks, holes and weak edges need attention before primer goes down. Use an appropriate repair compound as per the system specification.

 

Matching Epoxy Primer to Site Conditions

The right product for the right conditions, this is the principle that separates a professional installation from a failure waiting to happen.
The Arturo range addresses the 3 core site scenarios:

Standard epoxy primer: for sound, dry concrete and cementitious screeds within an acceptable RH limit below 75% RH

Epoxy DPM primer: for substrates with higher residual moisture above 75% RH, within the stated tolerance of the product.

Epoxy scratch coat: for rough or uneven surfaces that need pore-filling and smoothing before a self-smoothing compound is applied.

Porous concrete and cement screeds are the most common challenge floor installers face. An absorbent substrate, one with high open porosity, will aggressively absorb resin and water, which can lead to the following if not primed: 

• Pinholes and air bubbles as gas escapes from the substrate through an unsealed topping.
• Uneven curing caused by inconsistent moisture levels across the surface
• Poor bond strength 
• Surface texture inconsistencies and finish quality issues

A properly applied epoxy floor primer for concrete seals the substrate, binds loose particles, and creates a uniform, non-porous surface that coatings can bond to reliably. This is why even seemingly solid slabs should be primed, as porosity cannot be judged by eye alone.

Of all the applications for epoxy floor primer, self-smoothing systems are arguably where primer selection and application have the greatest impact on end results. Here’s why:

• Air trapped in an unprimed or poorly primed substrate will release through the floor as it flows and levels, creating pinholes that are both an aesthetic and functional defect.
• Non-uniform absorption causes the self-smoothing compound to flow inconsistently, leading to ridges, dry areas and uneven thickness.
• Poor adhesion at the primer substrate interface is the most common cause of debonding failures in self-smoothing systems.

The standard professional approach for a self-smoothing system on concrete is:

1. Mechanical preparation (shot-blast or grind)
2. Arturo epoxy primer, applied as above, within specified coverage rate. Or an Epoxy scratch coat, where the substrate is rough, porous, or the system spec calls for it
3. Self-smoothing compound, applied within the overcoating window.

For substrates with elevated moisture, use an Arturo epoxy DPM before the self-smoothing build-up. Always ensure residual moisture is within the DPM product’s stated tolerance.

Under-Preparation

Skipping or cutting corners on mechanical preparation is the single biggest cause of adhesion failures in resin flooring. Shot-blasting, grinding and vacuuming are not optional extras. If pull-off values don’t meet the system minimum, stop and address the substrate before priming.

Incorrect Mixing

Off-ratio mixing, too much or too little hardener, products that are a primer that either stays tacky or cures with compromised properties. Under-mixing leaves unmixed material that won’t cross-link. Weigh or measure accurately, and mix thoroughly every time.

Wrong Coverage Rate

Apply too thin and you’ll get poor sealing, dry patches and insufficient dust binding. Apply too thick, and you risk extended cure times, pooling in low spots and adhesion issues with the topping. Follow the data sheet coverage rate and apply evenly.

Ignoring Moisture

Applying a standard epoxy primer for a concrete floor over a substrate with moisture levels beyond its tolerance is one of the most common and costly mistakes in resin flooring. Test the substrate. If RH is elevated, use the correct Arturo epoxy DPM product. If levels exceed even the DPM’s range, the substrate needs to dry before any DPM is applied.

2-Component epoxy systems require careful handling. Uncured epoxy resin is a skin and eye hazard, so use nitrile gloves and eye protection, and avoid prolonged skin contact. Ensure good ventilation in all application areas, particularly during mixing and application. 

Clean tools promptly with the recommended solvent before the primer cures. Hardened epoxy cannot be removed by conventional solvents. Dispose of waste material, particularly mixed, uncured primer, in accordance with local regulations. Cured epoxy can typically be disposed of as solid waste, but always check current guidance.

Arturo’s range includes options with reduced VOC content for applications where environmental impact or working conditions are a priority; check individual product data sheets for details.