Repair mortars for a maritime fortress of the XVII th century

External renders covering facades of buildings placed in maritime zones suffer severe direct aggressions of the surrounding micro-environment, as they are subjected to the action of salty mist, strong winds, even to sea water (sprinkles and freatic water). The mortars of these buildings often present pathology associated to these particularly aggressive conditions and the repair mortars to use in interventions have to be selected taking into account this situation. In the present paper a study developed in Laboratorio Nacional de Engenharia Civil (LNEC) about the rendering mortars of a XVII th century Fortress nearby Lisbon, placed at the Atlantic Coast close to the Tagus river estuary is presented. A brief description of the building and its History is made, the main defects observed in renders are referred and experimental in situ applications of several different renders and paintings are described. The tests carried out to verify their efficacy and their compatibility with old masonry and environmental conditions are also presented. Intervention options are pointed out based on the observations carried out, the test results obtained and the existing conditions of work on site. This study is carried out in the scope of the Research Project partially financed by the Portuguese Foundation for Science and Technology (FCT) POCTI / HEC / 57723/2004 ‐ Lime renders conservation: Improving repair techniques and materials on architectural heritage (http://conservarcal.lnec.pt.).