PVA for priming – good practice or a bad habit?

Primers play an important role in preparing surfaces, prior to the application of cementitious tile adhesive. They help to reduce the porosity of high absorbent substrates, which prevents the rapid suction of water out of the tile adhesive when applied.

This in turn improves the bond strength at the interface between adhesive and substrate. Where thorough preparation of all substrates is vital to remove friable and weak surface materials, priming, as a part of this operation, aids in consolidating the surface. Due to reactions that occur between plaster-based substrates and cement based adhesives, correct priming acts as an interface between the two materials preventing direct contact and therefore preventing the risk of bond failure by reaction.

It is therefore important that when identifying the requirements for tiling, as much consideration is given to the choice of primer, if required, as is given to the choice of the correct adhesive or grout. Most tile adhesive manufacturers offer specially formulated primers based on acrylic polymers in the main. The recommendations for priming are a key part of the tile adhesive manufacturers’ instructions but often too little attention is paid to these recommendations by the user. It is common practice that PVA liquids are used as a universal alternative. Priming is an area of our industry that is too often misunderstood.

The market is flooded with PVA based products that contain varying levels of PVA polymer. In choosing a general purpose PVA for priming, the varying polymer levels will also vary the film thickness that remains on the surface when the primer has dried. The overall thickness of the dried film will depend on the choice of PVA product and the dilution rate suggested by the supplier. More importantly many PVAs have a tendency to re-soften once dried, when brought back into contact with moisture. This re-softening is the basis of many failures resulting from the use of PVA primers.

Unfortunately, the use of tile adhesive will introduce water to the dried film of PVA and as the cement based tile adhesive cures by hydration it will begin to harden, which in turn will create a stress on the interface with the primer. This crystallisation curing process taking place on a now softened and weak PVA background has a high potential for reducing the overall bond strength between the adhesive and the substrate. The end result is likely to be adhesion failure. The level of the failure depends on the type of PVA used and the film thickness of the PVA polymer left on the surface of the substrate. 

It will also depend on the application – vertical wall applications are more vulnerable as they carry a downward gravitational pull due to the combined weight of adhesive, grout and tile. This will add additional stress to the weak primer interface. However, the effect can still be found on horizontal flooring applications.

While PVAs offer an economic option, for the reasons described, they can considerably increase the risk of failure.

It is therefore advisable to follow manufacturer’s recommendations by choosing the correct primer, which will ensure first time successful fixing. At UltraTile we would always recommend the use of an acrylic based primer.

For more information, please call 01827 871871.