Slate walls - to sandblast or not to sandblast?

[FightingTheDamp]

We are renovating a slate cottage. Sadly, it had been rendered in cement some years ago, which had started cracking and trapping moisture, contributing to major damp issues inside. We have had the cement render stripped off, and the joints cleared out, ready for the lime render. Some remnants of the cement render remain on the face of the slate stones in some places. Our renderer usually sandblasts to get the remainder off before starting on lime, which makes total sense on a porous stone like sandstone… but is it really necessary on a non-porous stone like slate? I would prefer to avoid the cost and potential damage to any loose stone - unless sandblasting is absolutely necessary. Would be grateful for your thoughts.

 

[Stevers]

As long as the cement is well adhered, I also can't see any advantage to removing impervious cement from impervious slate. The important point is to get as much cement as possible out of the porous joints - which has been achieved.

My basement chimney breast had been treated with a cementitious slurry, I made sure the joints were clear, but was unable to remove all of the cement slurry from the tough old limestone it was built from. I just removed what I reasonably could and disrupted the rest to give it a key, for the NHL render that has worked perfectly - there's been good adhesion and it has dried out nicely ready for paint.