Hi, need some advice! I have a huge grey cement block garden wall, the wall is not rendered so it is rough, though i have painted a few coats over some parts of it. I want to mosaic directly onto it. What adhesive should i use? It needs to be very weather resistant, the wall is very high so gets full sun and wind and rain, temperatures between minus 11 °c in winter up to high twenties in summer. I have made a small mosaic out of pebbles just putting cement directly onto the wall this week- i hope this is weatherproof and doesnt crumble off!
So, should i use just cement for all the tile mosaics straight onto the wall? , will it last, will the adhesive or cement take differently to the painted part of the wall? and has anyone tried this before? Thank you!

 

The paint is going to make it difficult for anything to stick. The first step is to have the wall pressure washed to remove the paint. Then depending on how you are going to do the mosaic you might want to do a scratch coat of mortar on the wall before attaching the mosaic. For adhesive I would look at mortar or thinset depending on your mosaic.

 

Thanks for the advice! Since you seem to know a bit about this, could I ask one more question, I also have a small wall made of cement slabs slid into cement posts, the builder who put them in said not to bother painting them as he thought it would flake, could I put cement directly onto them to decorate them with a small mosaic tiles... they have a quite dusty smooth finish, thanks again for any help!

 

I would give the wall a thorough pressure washing or a good scrubbing with a wire brush and rinse to remove any loose material. With a clean, solid surface your mosaic will stick much better.

I don't know where you are located but here in NC we get frequent freeze/thaw cycles in winter. When doing stone veneer or mosaic I try to insure there are no air pockets behind any of the pieces and there is good solid mortar around the pieces to insure no water gets behind them.

 

My hubby had problems with this, strangely there were also some nails in the cement, don't know why... but he had to cut them off with a rotary cutting tool. He then smoothened the uneven surface with a thinset.