THE OPTIMUM PRIMER
F. Ball technical service manager Stephen Boulton provides expert advice on selecting the optimal primer for any project.
In nearly all cases, it is essential to prime a subfloor before the application of an adhesive or levelling compound. The reasons for this depend on whether the subfloor is an absorbent or non-absorbent surface, and this will also determine the best choice of product.
When applied over absorbent subfloors, including porous concrete and sand/cement screeds, primers prevent the unacceptably rapid drying of subsequently applied subfloor preparation products and adhesives and promote their adhesive characteristics. Insufficient water for complete hydration of a levelling compound may result in poor strength build up. Preventing the rapid loss of water from a levelling compound will also prolong its open time. Adhesives that dry too quickly can lose their tackiness and ability to adhere floorcoverings properly.
Another reason for priming is to prevent ‘pinholing’. This can be caused by the slow escape of air from absorbent surfaces, such as concrete or sand/cement screeds, which takes place while the levelling compound is curing and creates small air bubbles, which burst upon reaching the surface. This causes the appearance of pinholes or blisters once the levelling compound is fully cured.
There are general-purpose primers on the market, such as F. Ball’s Stopgap P131, 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. Typically, the first coat is applied dilute and the second coat neat. The contractor would normally use a roller for application to ensure even coverage of the primer, which is often coloured to make this easier to gauge.
When applied to non-absorbent surfaces, such as waterproof surface membranes, terrazzo or ceramic tiles, primers also serve to promote the adhesive characteristics of
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subsequently applied levelling compounds. For these purposes, specialist primers are available, such as F. Ball’s Stopgap P141, which are specially formulated to create a textured finish and enhance bond performance between subfloor and levelling compound.
This extra adhesion is particularly desirable where deep base levelling compounds are applied at a thickness greater than 20mm; the strong bond created when using Stopgap P141 is better able to withstand the increased stress that thicker levelling compounds experience when drying.
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. F. Ball’s Stopgap P121 primer is specially formulated to promote the application characteristics of compatible levelling compounds when applied over calcium sulphate screeds. It contains finer particles than others in the F. Ball range, helping it to bond with calcium sulphate screeds, which are denser than concrete or sand/cement screeds. The product should be applied in two coats; the first diluted with one part water to one part primer, followed by a second coat applied neat.
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% (or 65% where wood floorcoverings will be installed), 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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