Remove the old existing mortar. Clean the masonry with a vacuum cleaner, oil-free compressed air and water. A high-pressure cleaner should not be used, as this device will wash away the remaining relatively soft lime mortar. It is better to use a garden hose. In order for the mortar to adhere optimally, the substrate must be clean and free of dust and loose particles. All moss and lichen growth must be removed. 

Die Oberfläche muss vor den Mörtelarbeiten ausreichend vorgenässt werden, damit der Untergrund dem neuen Mörtel nicht zu viel Feuchtigkeit entzieht. Auch kann so das gelöste Kalziumhydroxid des Mörtels via Porenwasser in die Poren des Untergrundes eindringen. Dies erhöht den Verbund Mörtel-Untergrund.

The binder should fill all voids in the aggregate (sand) without pushing the individual grains of sand apart. The proportion of voids in a sand can be measured by adding water to dry sand until it begins to float.

If there is an excess of binder, the risk of shrinkage cracking increases. If there is too little binder, the mortar will not achieve sufficient strength.

Mixing ratios (volume) of 1:3 (England) to 1:5 (Germany) are mentioned for hot slaked putty mortar. 1:3 seems rather too "fat". A mixing ratio of 1:4 to 1:5 is realistic. It should be borne in mind that the volume of lime increases by a factor of 2 to 2.5 during hot slaking.

Rule: As little water as possible, as much water as necessary. More water leads to greater shrinkage. Test: Throw the mortar onto the bucket wall. If it adheres, the mixture is sufficiently smooth / moist. 

The largest grain of the sand used should not be larger than the smallest joint width of the masonry. The sand should have a good grading curve, i.e. it should have an even grain size distribution.

Ideally, lime mortar should be applied in thin layers. If this is not possible, the following aspects must be taken into account: 

The thicker the mortar pack that is installed, the greater the tendency for cracks to form. It is advisable to install as many additional bricks as possible in thick mortar sections (fitting whole bricks that are stable without mortar. If this is not possible, the stones are fixed with stainless threaded rods / stainless grids). It is generally better to apply the lime mortar in several thin layers. Hot slaked lime mortar is more suitable for the installation of thick mortar sections (when filling joints). Swamp lime mortar is more susceptible to the formation of shrinkage cracks. The surface of the applied lime mortar can be worked again in the first few days. If the surface is scraped, the surface is compacted again. Small shrinkage cracks are closed. The sintered lime skin that forms is removed and the grains of sand on the surface are exposed so that the color of the sand becomes visible.

For the application of lime mortars, the ambient temperature should be above 5°C.

All mortars must be kept moist after installation, depending on the weather conditions and location. This applies in particular to lime mortar (pure lime mortar or hydraulic lime mortar):

The hardening of the lime through carbonation takes several weeks and can only take place if there is sufficient moisture in the mortar. This means that freshly installed lime mortar must be protected from drying out (wind, sun). Normally, freshly mortared surfaces are covered (e.g. with jute) and moistened regularly.

The frost/de-icing salt resistance of mortar depends, among other things, on the pore volume in the mortar. A larger pore volume increases the frost/de-icing salt resistance. Salt and ice crystals have more room to grow before they destroy the structure of the mortar. One way to increase the pore volume is to add a porous aggregate: tuff sand.