What is a Primer?

Primer is used to treat a surface prior to tiling. Primers are designed not to coat the surface in any way, this is because they are an impregnator as opposed to a barrier. This helps with the following. 

1. Improvised mechanical grip 
2. Prevents problems such as ettringite failure (a chemical reaction between the adhesive and substrate that causes crystals to form until the tile is dis-bonded) 
3. Stabilises dusty substrates