Many people think that PVA is okay as a primer on walls and floors before tiling. This is not the case, as traditionally PVA is a multi-purpose product and not specifically formulated to work with tile adhesives.
First of all let’s think back, we all remember PVA – the glue you used in school to paint all over your hand, then see who could peel it back to get the biggest piece off! Peeled off easy? Washed off easy?
When you treat a surface with PVA it only partly soaks in and sits on the surface of the substrate much in the same way as wall paper paste. If PVA gets wet it becomes slightly live again, it doesn’t completely return to its liquid state but it becomes sticky.
When you spread tile adhesive onto a wall, the water in the adhesive makes the PVA live and stops the adhesive from penetrating the substrate and providing a mechanical grip. Basically your tiles, grout and adhesive are being held in place by a thin layer of PVA.
Tile adhesives work by crystallising when it sets. Once the adhesive starts to set crystals form and expand into any imperfections in the substrate (at a microscopic level) to create a grip. PVA stops this process by creating a barrier between the substrate and the tile adhesive. More...