Questions about tile adhesives

vax2002

I think the main reason for any primer is to seal the wall and prevent it sucking the moisture from the adhesive (improve open time) so it contacts correctly, the polished thing may be a bit of a red herring.

Leif

andyhop wrote: »

Any rapid set adhesive (which are all cement based) require primer when fitted to plaster walls

Lime(from the cement) and gypsum can form ettringate crystals which lead to a explosive reaction between the 2 substrates, Tiles will pop off and could cause injury

Thank you very much, that explains a lot. Several people have said they used a cement based plaster in some places, and that it needed primer for the reasons you give. So now I know it is the rapid set adhesive that is the cement based one.

Leif

vax2002 wrote: »

I think the main reason for any primer is to seal the wall and prevent it sucking the moisture from the adhesive (improve open time) so it contacts correctly, the polished thing may be a bit of a red herring.

I do not know for sure, but as you say I think the reason for the primer with some adhesives (and paint) is indeed to stop it drying too quickly, and thus not having time to soak into the substrate. If the substrate is rough then that does not matter because there is plenty of surface area. If the surface is smooth, then the adhesive needs time to soak in, and get hold of the substrate.

I tested this by putting neat paint on a smooth plaster surface. As expected the paint easily peeled off in large pieces. In fact when I removed the wallpaper from the walls in my house, I was able to take off almost an entire wall of paper in one go, and the paint came off with it, due I think to not applying a mist coat when painting. So in that case it was a good thing.