Jim Percival considers the importance and reasons for priming and sealing of substrates prior to tiling
SEALING and priming are important stages of a tiling project, without which the installation itself is at risk of failing. But it is important to understand the circumstances that dictate which procedure is required. Put simply, a tile fixer would prime a dusty surface, whereas he or she would seal a porous surface.
For example, if it is a concrete surface being tiled, it’s likely to be dusty, irrespective of whether it’s a new or old surface. The substrate will therefore require priming, or otherwise the tile adhesive will just stick to the dust.
Sealing, on the other hand is required for a new plaster substrate, because this is normally porous, almost like a sponge, so it should be sealed to prevent it absorbing all the liquid in the adhesive. There are products available which act as a primer, sealer and bonding agent.
These can seal anhydrite screeds and gypsum surfaces and can also be used to create a slurry as an admixture with a cementitious tile adhesive, enabling the tiler to coat impermeable surfaces to create the chemical bond for the tile adhesive. This approach is often used when installing large format or porcelain tiles externally to ensure a high-strength bond to the substrate.
You can have an SBR, which is Styrene-Butadiene Rubber, or an APD, which is Acrylic Polymer Dispersion. These are both water-based and essentially do the same thing. Generally an APD dries more quickly than an SBR.
Solvent-based primers (often epoxy-based) are generally used when maximum bond strength, extreme moisture resistance, or stabilisation of difficult, non-porous substrates is required.
While water-based acrylic and SBR primers are standard for most porous surfaces (like new plaster), solvent-based products are specifically for challenging, high-stakes, or commercial scenarios.
These types of epoxy resin primers are designed to create a separation layer. The reason for that is when you’re tiling on to an anhydrite screed, the calcium of the anhydrite reacts with the cement in the adhesive, creating deposits known as ettringite, which can cause expansion and cracking. You can achieve this with water-based sealers also, but good preparation is required, following the manufacturer’s instructions.
Whether to use a water-based or solvent-based primer is dependent on what the top surface coating is going to be. If you’re putting a resin paint down, a resin primer is generally used because it creates a better bond for the paint to go on.
Whatever the nature of any particular installation, getting this right is key to achieving a good tiling result. Preparation is key!
