We're talking about a house of the older type. At that time, perhaps no vapor barrier was used, or it may have disappeared over time.
Because of this, it is possible for the soil moisture to travel up in the walls. Problems arise when both the exterior and interior walls are painted with products that seales of the walls, which keeps the moisture inside and do not let it evaporate naturally. When salt gets wet inside the wall it crystallizes, it's like small explosions inside the wall, and the mortar and paint eventually fall off.
To be sure that moisture could get out of the walls, we had to ensure that there was no old paint left on the masonry. Therefore we chose to sandblast, not with sand, but with lava stone, which is not as hard as sand, in order to protect the old bricks.
The vicarage was treated with KefaTherm Exterieur, and since there have been no problems. The vicarage has a thatched roof which eventually gives dirt deposits on the walls, this problem is now solved with KefaTherm Exterieur who break the surface tension and therefore does not attract dirt in the same way as other paint products. When it rains, there is a cleaning effect, due to the micropores in KefaTherm Exterieur and their capillary effect.