No primer

[hmtiling]

Then we come back an age old question, how does gypsum plaster finish survive on sand/cement undercoats.

Pva sorts that one😉

 

[Old Mod]

Then we come back an age old question, how does gypsum plaster finish survive on sand/cement undercoats.

Now I've read about that quite recently, and I can't for the life of me remember where or what it said! Haha
Not a lot of help I know. But I'm trying to jog my memory. 🙄
But I seem to remember it works one way round, but not the other. 😕

 

[hmtiling]

Now I've read about that quite recently, and I can't for the life of me remember where or what it said! Haha
Not a lot of help I know. But I'm trying to jog my memory. 🙄
But I seem to remember it works one way round, but not the other. 😕

I think I remember hearing that the amount of gypsum in a skim coat isn't enough to cause a reaction.
I could also have made this up

 

[LM]

Then we come back an age old question, how does gypsum plaster finish survive on sand/cement undercoats.

I'm going by a site meeting I had several years ago where I was being asked to use Evo Stik OPF by the builder, who was suppling all materials. I wanted primer for the walls which he wasn't supplying, henceforth a debate started about it. The Evo rep said it wasn't needed, but I wasn't happy so I got a BAL/Ardex rep involved because I wanted to use their products. The site had 150 houses to be tiled and I wasn't for cutting corners. There was a site meeting with BAL, the building company and myself and the BAL rep went into it in detail explaining the chemistry of it to the point that the builder gave up and decided to pay more and run with them.
I've been on a few sites where cementitious adhesive has been used with no primer onto gypsum and there has been failures. When the tiles were removed the layers of dust (ettringite crystals) could be seen and they are what caused the separation.
For me on all my sites we prime, always!

 

[White Room]

I'm going by a site meeting I had several years ago where I was being asked to use Evo Stik OPF by the builder, who was suppling all materials. I wanted primer for the walls which he wasn't supplying, henceforth a debate started about it. The Evo rep said it wasn't needed, but I wasn't happy so I got a BAL/Ardex rep involved because I wanted to use their products. The site had 150 houses to be tiled and I wasn't for cutting corners. There was a site meeting with BAL, the building company and myself and the BAL rep went into it in detail explaining the chemistry of it to the point that the builder gave up and decided to pay more and run with them.
I've been on a few sites where cementitious adhesive has been used with no primer onto gypsum and there has been failures. When the tiles were removed the layers of dust (ettringite crystals) could be seen and they are what caused the separation.
For me on all my sites we prime, always!

I've seen jobs where cement based adhesive has been applied to gypsum plaster and there's never been a problem.

 

[LM]

I've seen jobs where cement based adhesive has been applied to gypsum plaster and there's never been a problem.

So have I in the past and I can't explain it, but I'm no chemist and I'm not taking the risk for the sake of priming a wall 🙂

 

[White Room]

Me neither🙂

 

[Aston]

Just use your common sense and experience and if you don't have either of them, go with this old saying, "if in doubt prime"

 

[SJPurdy]

Then we come back an age old question, how does gypsum plaster finish survive on sand/cement undercoats.

As I understand it the ettringite is formed during the hydration phase of the portland cement. If the sand cement is already set then skimming over it with plaster should be ok because it is only the plaster that is setting.
But if the wet Portland cement mix is applied over a set plaster surface then the ions required for the formation of the ettringite can be taken from the plaster surface while the cement is hydrated and so the crystal ettringite layer can form where the two meet; giving a weaker bond.