I suppose what irish_spread's been saying is that its only a problem if its a problem.
We use a product called linostone for cosmetic repairs on sandstone, also to tie in cement render /blockwork extensions etc to existing sandstone. It was big time here in the 70's 80's stonecleaning free for all. But it can trap moisture in a soft wet/dry cycle material, is not breathable, can further corrodes sandstone inversely in a wick effect & in the wrong hands looks like a pantomime house. Its vilified by historic Scotland & conservation groups....but we've/our clients have never had a problem with it, because I use my experience judging when to use it(like for like repairs etc). Hard impervious(not porous) repairs & materials are good as a sealed unit, that's the key imo.