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Choosing the optimum primer

Neil Sanders discusses the importance of priming subfloors, and how contractors can ensure they select the right product

WITH few exceptions, it is essential to prime a subfloor before the application of an adhesive or levelling compound. The reasons for doing this depend on whether the subfloor is an absorbent or non-absorbent surface, and this will also determine the best choice of primer.

When used on non-absorbent surfaces, such as waterproof surface membranes, terrazzo or ceramic tiles, primers serve the important role of promoting adhesion between the subfloor and most levelling compounds applied over it.

When applied over absorbent subfloors, including porous concrete and sand/cement screeds, primers promote adhesion, and they also create a film to prevent the unacceptably rapid drying of a subsequently applied levelling compound or adhesive. If it dries too quickly, a levelling compound will not perform to its optimum capability. Adhesives that dry too quickly can lose their tackiness and ability to adhere floorcoverings properly.

Another main reason for priming is to prevent pinholing. Pinholing can be caused by the slow escape of air from absorbent surfaces, such as concrete or sand/cement screeds, which takes place while the floor levelling compound is curing. This creates small holes in the levelling compound that have the appearance of pinholes or blisters once fully cured. By applying a primer, a film is created across the surface of the floor, preventing this airflow and reducing the risk of pinholing.

The application of a primer will also reduce the absorbency of the substrate and prevent the rapid release of moisture from the levelling compound. This will lead to poor strength build-up in the levelling compound because of insufficient water for complete hydration.

There are general-purpose primers on the market that can be used over both absorbent and non-absorbent surfaces.

However, if preparing a particularly absorbent subfloor, it may be necessary for a contractor to apply a second, or even third, coat of the primer. Typically, the first coat is applied dilute and the second coat neat. The contractor would normally use a roller for application onto a non-absorbent surface, to ensure even coverage of the primer, which is often coloured to make this easier to gauge.

Primers are available for a number of other specialist applications. One common cause of concern amongst flooring contractors is installing floorcoverings over calcium sulphate screeds, which are a popular choice in many new builds. A chemical reaction between calcium sulphate screeds and ordinary levelling compounds can cause the formation of ettringite; a crystalline material that can cause floor failure. It is for this reason that F. Ball recommends a calcium sulphate-based levelling compound is used over calcium sulphate screeds; to avoid any potential of this reaction occurring. For these purposes, specialist acrylic primers are available, which promote the application characteristics of calcium sulphate-based levelling compounds.

F. Ball has also recently launched a primer especially developed for use over non-absorbent surfaces, such as power floated concrete, ceramic tiles and epoxy waterproof surface membranes. Stopgap P141 has been formulated to create a textured finish and enhance bond performance between subfloor and levelling compound. This extra adhesion is suitable where deep base levelling compounds are applied at a thickness greater than 20mm. The strong bond created when using P141 is able to withstand the increased stress that thicker levelling compounds experience when drying. Applied neat with a paint roller, five litres of Stopgap P141 will cover an area of up to 65 square metres. It is coloured yellow.

A final word on subfloor preparation: before applying a primer, the contractor should make sure the subfloor is suitably prepared, dry and free of contaminants. A moisture measurement test should also be conducted to determine levels of subfloor moisture. If relative humidity (RH) levels are above 75%, a damp proof membrane will be required to suppress excess moisture and provide a barrier which will prevent moisture damage to the floorcovering or could even cause complete floor failure.

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