External renders based on air lime are used in historical buildings as decorative and protective coats. They act as sacrificial layers, particularly exposed to climatic actions and mechanical and environmental impact. They generally present good cohesion and adhesion to the background, although often suffer some degradation as detachments, superficial cracks, or 'lacunae'. Render solutions specified today for old buildings repair are frequently incompatible with pre-existent materials and inappropriate for the specific situation, thus producing new pathology. The choice of adequate materials, formulation, and curing conditions of substitution mortars become crucial for the success of the conservation action. These mortars must meet functional and aesthetic requirements, to fulfill the principles of compatibility, maintaining buildings' integrity and durability. An analysis is presented of ranges of results for different lime-based mortars for old buildings external renders, obtained in several studies carried out at LNEC. The mortar formulations studied include: air lime, air lime and cement, air lime and hydraulic lime, air lime and pozzolans, and air lime and other additions. Their main physical and mechanical characteristics are analyzed and compared. Examples of case studies are presented and analyzed. Their adequacy to be used in historical buildings and possible application fields are discussed.