The importance of priming a sub floor

F. Ball and Co. Ltd.’s Technical Service Manager, Stephen Boulton, gives guidance on priming, an essential but often-overlooked part of subfloor preparation.

Along with poor management of subfloor moisture, the absence of priming is one of the most common causes of failure in new flooring installations. Priming can be perceived as time-consuming, costly or even unnecessary, but F. Ball recommends that primers are essential to a successful installation. Primers are designed to promote the adhesion of smoothing underlayments to both absorbent and non-absorbent surfaces. Primers cannot act as stand-alone surface preparation systems, or correct poor quality substrates, but can reduce the risk of surface imperfections occurring in new installations.

The application characteristics of flooring installation products will be enhanced by the use of primers. Using a primer to seal the porous surface of a subfloor reduces surface tension and promotes adhesion by improving the flow properties of any smoothing underlayments being applied. Priming will help to reduce water loss into the substrate when using underlayments such as cement-based smoothing underlayments, which will retain a ‘wet edge’ longer for ease of installation. Priming will also add to the strength and reduce the shrinkage characteristics of smoothing underlayments being applied. Using a primer to reduce the amount of water lost will extend the open time of flooring adhesives being applied to concrete surfaces and porous screeds.

As with all aspects of subfloor preparation, the condition of the subfloor surface and type of underlayment that will be applied should be taken into account and will enable the most appropriate type of primer to be selected. Acrylic primers, such as F. Ball’s Stopgap P121 for example, are recommended to promote the application characteristics of smoothing underlayments over calcium sulphate screeds. More general purpose primers, such as Stopgap P131, can be used for a wider range of absorbent and non-absorbent surfaces, with certain primers being approved for use in marine applications as part of an IMO system.

The selection of an inappropriate primer or failure to follow manufacturer instructions can lead to the failure of a subfloor installation. Applying an uneven coat of primer is another cause of failure for installations, which is why many primers are pigmented with colour to provide a visual aid to coverage control. Cracking of an underlayment or even failure to bond correctly to the subfloor can occur as a result of incorrect priming, creating a potential hazard and being likely to result in costly remedial work.

For a primer to work effectively, the subfloor surface that it is applied to must be smooth, clean and dry. Where excessive moisture levels exist in a subfloor, an epoxy resin system must be applied to create an effective dampproof membrane before applying a primer.

The temperature of a subfloor is also important for the correct application of primer. Most primers require that the temperature of the surface must be at least 5°C before a primer can be applied, and in many cases the minimum required temperature is 10°C. Where an underfloor heating system is present F. Ball recommends that the system is switched off for 48 hours before, during and after the application of a primer, enabling the product to dry at a natural rate.

In addition to the performance benefits that priming offers for absorbent and non-absorbent subfloors, priming will also enable some smoothing underlayments to be applied over specific types of contaminated surfaces. Stable adhesive residues can be primed where only minimal amounts are present, although caution is advised when priming surfaces that could contain silicon-based treatments, as this can inhibit bond formation. The F. Ball technical team are always available for contractors that need advice on selecting the most appropriate primer system for use over contaminated subfloors.

Priming should be considered a standard part of any subfloor preparation procedure for contractors, promoting the durability of a flooring installation. Priming improves bond strength and reduces the risk of surface imperfections. Primers are easy to apply and offer long-term benefits to every flooring installation.

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