Priming is often a step that is overlooked during the preparation of asubfloor for the installation of floor coverings. It may be perceived as unnecessary, costly or time-consuming, but priming can be the key to a successful flooring installation.

Importance of preparation

Preparation is of the utmost importance: From the way you plan your application, to the necessary tools for the perfect floor to using the adequate vacuum grinders fit for the purpose. Focus on the actual application from where to start to where to finish.

  • Must the joints be filled, re cut and re sealed?
  • Do you have acetone or a super brush cleaner to clean tools if necessary?
  • Where will you clean your tools?
  • Do you have access to electricity and water?

Sealing the substrate is the second most important part of any application. The main reason for this is to completely seal the substrate, it will help to eliminate any `outgassing` (tiny bubbles). This is one of the main reasons why a second cote of sealer is advised.

Reasons for priming a subfloor

Subfloors should be primed before the application of an adhesive or a levelling compound, unless working with flooring preparation products that are specifically formulated to not require the use of a primer.

The reasons for this depend on the type of subfloor:

  • Enhancement: When applied to non-absorbent surfaces – such as waterproof surface membranes, terrazzo or ceramic tiles – primers are designed to enhance the application characteristics of subsequently applied levelling compounds.
  • Promote adhesion: Priming absorbent subfloors, including porous concrete and sand/cement screeds, promotes the adhesion of subsequently applied flooring preparation products, as well as preventing the unacceptably rapid drying of levelling compounds and adhesives. Insufficient water for complete hydration of a levelling compound may result in poor strength build-up.
  • Consistent drying: Avoiding the rapid loss of water from a levelling compound will also prolong its open time. Similarly, adhesives that dry too quickly can lose their tackiness and ability to adhere floorcoverings properly.
  • Prevention of surface defects: Primers can also prevent the incidence of surface defects, such as “pinholing”. It is caused by tiny bubbles resulting from the slow escape of air from the subfloor, which burst on reaching the surface, leaving small pinholes or craters as the levelling compound cures.

Priming time

The reason for priming depends on the type of subfloor, as this will determine the optimum primer for a particular situation. General purpose primers are available that can be used over both absorbent and non-absorbent surfaces. If preparing a particularly absorbent subfloor, it may be necessary for a contractor to apply a second or even third coat of the primer. The contractor would normally use a roller for application to ensure even coverage of the primer, with pigmented primers making this easier to gauge visually.

For non-absorbent surfaces, specialist primers are available that are designed to create a textured finish and enhance the bond performance between the subfloor and levelling compound. This extra adhesion is particularly desirable where deep-base levelling compounds are applied at a thickness greater than 20mm.

Primers are also available for specialist applications, such as when working with calcium-sulphate screeds. These are formulated to promote the application characteristics of compatible levelling compounds – such primers contain finer particles than other products, helping them to bond with calcium-sulphate screeds, which are denser than concrete or sand/cement screeds.

Avoid weak points

When contractors are working over raised-access panels, the direct application of a levelling compound to the substrate may result in weak points, where the levelling compound sinks into the channels at the joints between panels. In cases like these, cement-based primers must be used to fill the joints between panels.

When working with levelling compounds that can be applied directly over well-bonded old adhesive residues, the subfloor doesn’t require priming.

Full acknowledgement and thanks go to for the information in this editorial.

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