Stabilising a sandy screed

[Donwill]

I have a sandy screed the surface of which is powdery in nature i.e there are sand grains becoming detached despite (or because of )repeated brushing with a soft brush. I need to lay ceramic tiles on the screed and require to apply some liquid which will penetrate the top layers of the screed hopefully by capillary action to some depth, and setting to hold the sand in place. Obviously the liquid should not be very viscous and should have a suitable surface tension, is there some substance which will do this? possibly some acrylic glue of some kind, I don't think that a PVA based substance would do as in my limited experience it's may be a bit too viscous. What does the team think

Cheers Don

[Grunff]

Yes, there is.

PVA would work great for this, provided you dilute enough. You have a lot of control over the viscosity. Dilute some at 1:4 in water, and the viscosity will only be very slightly higher than water. It will penetrate very well.

[The Wanderer]

I had exactly the same problem when I extended my conservatory and then wanted to tile right through - the screed in the original part was just as you describe. The original part had had a foam-backed carpet laid, and that also brought about some problems in that the foam backing had deteriorated in part to dust as well, and was also sticking in patches on the screed.

I used unibond. I worked a small area at a time, sort of half a dozen tiles at at a time. I had to use a paint scraper to get rid of the old foam in places, then brushed and lightly vacuumed it clear. I used unibond watered down to about 50% and applied just before I spread the adhesive. The Unibond soaked straight into the screed.

It worked a treat, the tiles are well-bedded and I've had no signs of problems since it was done about 5 years ago.

[The Natural Philosopher]

Thinned PVA. Dilute 50/50 with water.

It works a treat and is in fact recommended before tiling anyway.

[Phil L]

As others have said, PVA diluted 1:1with water, this will soak right in giving you a better surface to work with, you can then give it a second coat diluting it only 1 part water to 2 parts PVA, yes this is more viscous but the surface will already be partially sealed with the first coat.

[Donwill]

Many thanks for the info!!

Regards Don