How to Choose an Epoxy Floor Primer for Concrete

For dry, porous concrete requiring rapid recoating, Fortis 823 Prime is generally the preferred option. Fortis 820 Prime is designed for difficult or porous concrete where high adhesion and substrate consolidation are priorities. Where significant moisture transmission or rising damp is present, a suitable moisture-control system such as HydroFort VB may be required.

Some epoxy coatings can also self-prime properly prepared concrete, eliminating the need for a separate primer where permitted by the coating specification.

Which Epoxy Primer Should You Use?

The following comparison provides a starting point for selecting a Fortis epoxy primer. Final selection should be based on substrate assessment and the current product Technical Data Sheets.

Moisture-tolerant primers and moisture vapour barriers serve different purposes. Fortis 820 Prime should not be used as a substitute for a specified vapour barrier.

When Is an Epoxy Floor Primer Necessary?

A separate epoxy primer is not always required. Some epoxy flooring systems can be applied directly to properly prepared concrete, with the first coat acting as a primer.

However, a dedicated primer may be beneficial or necessary when:

• Concrete is highly porous or unevenly absorbent.
• The substrate requires additional penetration or consolidation.
• Improved adhesion is required on a difficult surface.
• The flooring system specifies a separate primer.
• Rapid recoating is important to the project schedule.

Priming does not compensate for inadequate surface preparation, contamination or structurally unsound concrete.

Fortis 823 Prime — Fast-Curing Epoxy Primer

Fortis 823 Prime is a two-component solvent-based epoxy primer formulated for penetration and wetting of properly prepared concrete.

Its fast-curing characteristics make it suitable for projects where efficient recoating is important, including garages, workshops, warehouses and decorative flake flooring installations.

When Should You Choose 823 Prime?

• Concrete is dry, sound and mechanically prepared.
• The substrate is porous and requires sealing before coating.
• Fast recoating and project turnaround are important.
• The subsequent coating is compatible with 823 Prime.
• A decorative flake or high-build epoxy flooring system is being installed.

Highly absorbent concrete may require additional wet-on-wet primer application to achieve a uniformly sealed surface.

823 Prime is commonly used beneath compatible systems such as Fortis Coat 823 HS.

Fortis 820 Prime — High-Adhesion Epoxy Primer

Fortis 820 Prime is a two-component high-solids epoxy primer designed for concrete substrates where adhesion, penetration and consolidation are important.

It is particularly useful for porous or difficult concrete and may be applied to suitably prepared slightly damp substrates where there is no standing water, subject to the current product specification.

When Should You Choose 820 Prime?

• Concrete is porous, difficult or unevenly absorbent.
• Additional substrate consolidation is required.
• High adhesion is a priority.
• The concrete is slightly damp but otherwise suitable for coating.
• A compatible high-build epoxy flooring system will be applied.

820 Prime does not eliminate the need to remove weak concrete or correct moisture transmission problems.

823 Prime vs 820 Prime — What Is the Difference?

Both products are epoxy primers, but they are designed around different application priorities.

Choose 823 Prime where rapid recoating is the main priority on suitable dry concrete. Choose 820 Prime where difficult substrate conditions and high adhesion are the primary considerations.

What Is the Best Epoxy Primer for Damp Concrete?

The correct product depends on whether the concrete is slightly damp or subject to ongoing moisture transmission.

For suitably prepared slightly damp concrete without standing water, Fortis 820 Prime may be appropriate. However, where rising damp, hydrostatic pressure or significant moisture vapour transmission is present, a conventional epoxy primer may not provide sufficient protection.

In these situations, a moisture-control system such as HydroFort VB should be assessed.

When Should You Use HydroFort VB?

HydroFort VB is a two-component water-based epoxy vapour barrier designed to control moisture transmission through suitable concrete and cementitious substrates.

It is self-priming on appropriately prepared surfaces and may be used on damp concrete where the substrate is not saturated and standing water is absent.

Typical applications include:

• Concrete affected by rising damp.
• Floors requiring moisture control before compatible coatings.
• Concrete and masonry subject to moisture transmission.
• Preparation beneath compatible waterproofing systems.

HydroFort VB is normally applied as a two-coat system. Application rates, recoat times and compatibility requirements must follow its current Technical Data Sheet.

Where active water ingress or severe hydrostatic conditions are present, investigate the source and confirm that the complete system is suitable before proceeding.

Can Epoxy Floor Coatings Be Self-Priming?

Yes. Some epoxy floor coatings can act as their own primer on properly prepared concrete.

For example, suitable Fortis AquaCoat and Coat 823 HS systems may be applied using a self-priming first coat where permitted by their technical specifications.

Self-priming is generally most appropriate where the concrete is sound, correctly prepared and within the coating system's specified moisture limits.

A separate primer may still be necessary where the substrate is highly porous, difficult or requires additional adhesion performance.

How to Prepare Concrete Before Applying Epoxy Primer

Surface preparation is essential regardless of which epoxy primer is selected.

1. Inspect the concrete for damage, weak areas and contamination.
2. Investigate moisture conditions and potential rising damp.
3. Remove oil, grease, curing compounds and other contaminants.
4. Mechanically prepare the surface using suitable diamond grinding or shot blasting.
5. Remove laitance and unsound surface material.
6. Repair cracks, holes and damaged areas using compatible materials.
7. Vacuum thoroughly to remove dust and debris.
8. Apply the selected primer in accordance with its Technical Data Sheet.

The required surface profile depends on the primer, coating thickness and service conditions.

For more information on adhesion problems, see Why Does Epoxy Flooring Delaminate?

Common Epoxy Priming Mistakes

Even a suitable primer may fail if application requirements are not followed.

• Applying primer over dusty or poorly prepared concrete.
• Failing to identify contamination before grinding.
• Using a conventional primer where a moisture barrier is required.
• Applying insufficient primer to highly absorbent concrete.
• Mixing epoxy components at the incorrect ratio.
• Applying material outside the specified temperature conditions.
• Exceeding the maximum recoat window without appropriate preparation.
• Applying incompatible subsequent coatings.

Incorrect priming can contribute to coating defects, pinholes and delamination.

See Why Do Bubbles and Pinholes Form in Epoxy Flooring? for further troubleshooting information.

Frequently Asked Questions

What is the best epoxy primer for concrete floors?

The best epoxy primer depends on concrete porosity, moisture condition and the required coating system. Fortis 823 Prime is suited to fast recoating on suitable dry concrete, while 820 Prime is designed for difficult or porous concrete where high adhesion is important.

Which epoxy primer cures fastest?

Fortis 823 Prime is designed for fast curing and rapid recoating. Actual recoat time depends on temperature, application conditions and the current Technical Data Sheet.

Can epoxy primer be applied to damp concrete?

Some epoxy primers tolerate limited residual dampness. Fortis 820 Prime may be used on suitably prepared slightly damp concrete without standing water, subject to its specification. Ongoing moisture transmission may require a dedicated vapour barrier.

What is the difference between an epoxy primer and a vapour barrier?

An epoxy primer primarily improves adhesion and seals porous surfaces. A moisture vapour barrier is specifically designed to control moisture transmission that could affect subsequent flooring systems.

Does epoxy primer stop rising damp?

Not necessarily. Conventional epoxy primers should not be assumed to prevent rising damp. A specified moisture-control system such as HydroFort VB may be required.

Can I apply epoxy flooring without primer?

Some epoxy flooring systems can self-prime properly prepared concrete. Whether a separate primer is needed depends on the substrate and the manufacturer's system specification.

Can I apply epoxy primer over an existing coating?

Only where the existing coating is compatible, sound and firmly bonded. The surface may require cleaning, mechanical abrasion and adhesion testing before application.

What happens if epoxy primer is not applied correctly?

Incorrect priming can result in uneven absorption, pinholes, poor adhesion, blistering or coating delamination. Surface preparation and correct product selection are essential.

Related Fortis Technical Resources

• Why Does Epoxy Flooring Delaminate?
• Why Do Bubbles and Pinholes Form in Epoxy Flooring?
• How Many Coats Does an Epoxy Floor Need?
• Fortis Epoxy Floor Coatings

Need Technical Assistance?

Selecting an epoxy floor primer requires consideration of concrete condition, moisture, surface preparation and the intended flooring system.

Fortis Adhesives & Coatings manufactures epoxy primers and floor coatings in Melbourne, Australia, and can assist with product selection and system compatibility.

For advice on selecting an epoxy primer or complete flooring system, contact the Fortis technical team.

Technical note: This guide provides general product-selection information. Always refer to current Fortis Technical Data Sheets and assess site conditions before application. Moisture-affected or structurally unsound substrates may require specialist investigation.