Basic limewash applied to a stone vaulted ceiling. 

Sulfate staining and dusting due to poor specification of the limewash.

Inappropriate for this type of sandstone. 

The use of an air/pure lime is not recommended in such a sheltered, external application.

Solution; fine pozzolanic red brick dust added to the limewash. There are plenty of other additives and procedures that could be utilised such as poulticing the salt out first or using fresh cow muck or blood instead of brick dust. 

Not very complicated. The porosity of the stone is completely different. This is due to people who aren't familiar with stone specifying or cost cutting. I believe both types of stone are Portuguese sandstones...we have extensive data on British stone but its more expensive. 

Potential rapid deterioration of stone

More slippery

Aesthetically unpleasant

Solution; don't be tight, spend more on the specifier and the stone

Lime render in a sheltered location not drying out effectively and staining with biological growth. 

Cold damp property

Reduced lifespan

Aesthetically unpleasant

Solution: close it with a steel or magnesium trowel instead of sand floating. Or limewash it.