Do I Need to Prime My Subfloor Before Using a Levelling Compound?

Before applying a levelling compound, one of the most common questions is: do I need to prime the subfloor first?

The answer is not always. Whether a primer is required depends on the type of subfloor you're working with and the levelling compound being applied.

Some products can be applied directly to certain substrates, while others require a specific primer. This is why it's important to identify your subfloor and check the technical data sheet for your chosen levelling compound before starting.

Why Is a Floor Primer Used?

Primers prepare the substrate for the products being applied on top.

On porous and absorbent surfaces, a primer can regulate the porosity of the substrate. This can improve the workability, adhesion, strength and surface finish of subsequently applied levelling compounds.

On non-porous surfaces, a bond-promoting primer can instead create a textured, keyed surface for the levelling compound to adhere to.

Which Primer Do I Need?

There isn't one primer that's suitable for every subfloor. Kelmore, for example, offers different PrimeMore products according to the type of background.

Subfloor Type	Primers	Purpose
Concrete & cement:sand screeds	PrimeMore Universal	Regulates the porosity of absorbent backgrounds
Asphalt, epoxy DPMs & other specified non-porous backgrounds	PrimeMore Grip	Creates a textured, keyed surface to promote adhesion
Calcium sulphate/anhydrite screeds	PrimeMore CS	Seals the surface and creates a barrier before cementitious flooring compounds
Moisture control / substrate consolidation	Ardex PU 30	Primer, moisture barrier and substrate consolidator

Always check the technical data for both products before application, as requirements can vary depending on the levelling compound.

Priming Concrete & Cement Screeds

For porous backgrounds such as concrete and cement:sand screeds, Kelmore PrimeMore Universal is designed to regulate porosity before levelling and smoothing compounds are applied.

PrimeMore Universal is diluted 1 part primer to 3 parts water for the applications specified by Kelmore and has an approximate drying time of 30 minutes at 20°C.

Depending on the porosity of the background, additional coats may be required.

What About Non-Porous Subfloors?

Non-porous surfaces require a different approach.

Kelmore PrimeMore Grip is a ready-to-use, one-coat bond-promoting primer designed for non-porous backgrounds. It dries to create a textured, keyed surface onto which compatible levelling and smoothing compounds can bond.

Kelmore specifies PrimeMore Grip for backgrounds including flooring-grade asphalt, epoxy DPMs, steel and existing tile flooring, depending on the levelling compound being used.

At 20°C, it has an approximate drying time of 45 minutes.

What About Calcium Sulphate Screeds?

Calcium sulphate and anhydrite screeds require particular attention before applying cementitious products.

Kelmore PrimeMore CS is specifically designed for these substrates. It penetrates and seals the surface, creating a protective barrier that prevents adverse reactions with subsequently applied cementitious adhesives and flooring compounds.

PrimeMore CS is ready to use, requires one coat and has an approximate drying time of 60 minutes at 20°C.

Ardex PU 30 – More Than a Standard Primer

For projects that require more than a conventional primer, Ardex PU 30 is a one-component polyurethane product that can be used as a primer, moisture barrier and substrate consolidator.

As a primer, PU 30 can be used on a wide range of absorbent and non-absorbent substrates, including concrete, cement screeds, calcium sulphate screeds, terrazzo, ceramic tiles, chipboard, OSB and firmly bonded adhesive residues.

It can also be used to consolidate absorbent substrates and as a moisture barrier on cement screeds and concrete where elevated residual moisture is present, subject to Ardex's specified moisture limits and application requirements.

Key features include:

• Ready to use
• One-component polyurethane formulation
• Water and solvent free
• Suitable for absorbent and non-absorbent substrates
• Can consolidate absorbent surfaces
• Can act as a moisture barrier
• Suitable for use with underfloor heating
• Reworkable after approximately 1 hour

Because PU 30 can perform several different functions, the application method and number of coats depend on how it is being used. Always follow the current Ardex Technical Data Sheet for the specific substrate and application.

Does Every Levelling Compound Need Primer?

No – and this is why checking the product specification matters.

For example, Kelmore LevelMore Pro can be applied to several substrates without a primer, including flooring-grade asphalt, existing ceramic, porcelain and natural stone tiles, plywood overlays and tile backer boards.

On concrete and cement:sand screeds, Kelmore states that priming isn't necessary for LevelMore Pro to bond. However, PrimeMore Universal is recommended because regulating the substrate's porosity improves adhesion, workability and the finished surface.

Other Kelmore compounds have different requirements, so don't assume the same preparation applies across the entire range.

Final Thoughts

So, do you need to prime before applying a levelling compound? Sometimes – but not always.

The correct approach depends on:

• The type of subfloor
• Whether it's porous or non-porous
• The levelling compound being used
• The manufacturer's specified preparation method

Identifying the substrate first and following the levelling compound manufacturer's technical data will help you determine whether priming is required and which primer to use.

At Floormart, we stock the Kelmore PrimeMore, LevelMore ranges and Ardex PU 30 providing solutions for porous, non-porous and moisture-affected substrates.