calibrated slate on sand/cement screed - prime surface ?

[simonjay]

I've got some very well calibrated brazilian slate 400 x 600 x 10mm. The back is lightly grooved from the machining.
I also have some Dunlop natural stone and large format adhesive.
The substrate is sand/cement screed well dried (many months old) which is slightly dirty with a few paint splashes on etc. Not very dusty, but SOME light dusting if scraped, etc (like any screed).
Trowel I will use is large format trowel (20mm x 13mm rounded notch at 28mm spacing)

The question is:
Do I need to prime the surface before tiling ?
If so, with what ?

Thanks,
Simon.

 

[Gazzer]

I would use SBR after a good scrape and clean of floor.

 

[simonjay]

Would you use it neat or dilute it ?

 

[Gazzer]

3-1 ratio.( 3 water 1 SBR) or as instructions on product. At least one coat maybe 2 depending on how porous the floor is.

 

[Rich]

Spot on advice from Sir Ramic. Give the floor a good scrape/sand to get as much of the paint and snot off, then hoover it off and prime as to the instructions on the bottle and leave to dry. Also make sure that you back butter the tiles before pressing them down into the adhesive bed

 

[simonjay]

Yes, the back buttering is the usual thing. Interestingly, the dunlop natural stone and large format adhesive says it is a "new thing" and specifically designed to not require back buttering since it is designed to collapse in a way that guarantees fill coverage. They only recommend back buttering for tiles profiled or studded on the back.
Still, a thin back buttering could do it no harm !
Simon.

 

[Rich]

Aha, this is a full contact adhesive, like a pourable thick bed? Haven't used it myself.

 

[simonjay]

It is a pourable thick bed. I have not used it before either. I'll let you know how it goes
Simon.