noseall
with respect
what a load of ******s ...
old mate
...wrote this an Im sharing his Love an saving me tapping away like a monkey here ..
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"We had Ardex Technical down to the site to compile a report, the basis of which was it's the PVA that causes the problem.
When you treat a surface with PVA it partly soaks in and parlty sits on the surface of the substrate much in the same way as wallpaper paste.
If PVA gets wet it becomes slightly live again, it doesn't completely return to it's liquid state but it becomes sticky.
When you spread tile adhesive onto the wall, the water in the adhesive makes the PVA live and stops the adhesive from penetrating the substrate and providing a mechanical grip.
Most tile adhesive works by crystalising when it sets (some are slightly different such as epoxy based ones) but generally they all work the same way. Once the adhesive starts to set crystals from and expand into any imperfections in the substrate surface (at a microscopic level) to create a grip. PVA stops this process by creating a barrier between the substrate and the tile adhesive. Basically a moist layer of PVA is then holding the enitre weight of the tiles, grout and adhesive and stopping a great deal fo the mechanical grip occuring.
Ok so whats the difference between this and Ardex or BAL primer, well basically the tile manufacturers primers are generally acrylic (such as BAL APD- Acrylic Polymer Dispersant), the chemicals don't become "Live" when a tile adhesive is applied to it and stop the mechanical crystallisation of the adhesive gripping the substrate itself. They also stop a chemical reaction occuring between the cement based adhesive and a plaster substrate, a known problem know as "Ettringite failure"
BAL and some other manufacturers also produce SBR primers, these are slurry primers for flooring, so if you're about to tile a dusty sand/cement screed, a thin coat of SBR will bond all the surface dust together and allow the adhesive to bond to the screed correctly, SBR shouldn't be used as a wall plaster primer as a replacement of APD.
I hope this clears up any misunderstandings."
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I hope to have another good night on the lash with this fella if ever down his way again ...