Investing in a good quality VMC
Once your wall is dry, invest in high-performance ventilation. What for? Because the saltpeter only settles in humid atmospheres in which air does not circulate.
You have several choices depending on your budget:
VMC simple flux: Extracts the stale air from the humid parts and releases it outward via the ventilation ducts. The new air naturally enters through aerations located in the living rooms (so it is not aspirated)
VMC dual Flux: functions as a single-flow VMC, but also has the function of vacuuming and reheating the outside air to inject it into the building.
VMR (Distributed mechanical ventilation): Same operating principle as VMC, but air is expelled via individual aerators.
VMI (Reverse mechanical Ventilation): infuses hot air into the rooms.
Make the stains of saltpeter disappear
Once the two previous steps have been completed, you can proceed with the removal of the saltpeter. To do this, you remove all contaminated coatings.
Wait until the wall is completely dry to install a new coating. Depending on the humidity level, it may take several months.