Cement plasters covering exterior walls, besides being exposed to weather conditions, are also subject to the development of organisms like cyanobacteria, microalgae, fungi and lichens, that apart from the aesthetic impact, may deteriorate the material by physical and chemical mechanisms. A good cleaning process must eliminate these microorganisms but avoid damage to the material and include treatments that delay the growth of new colonies. With this aim, different cleaning mechanisms were tested on a wall covered with a stone imitation white mortar colonized by different organisms from a building in La Plata city built in the 1930s, in order to find the best cleaning method that delays for as long as possible the growth of new colonies. Samples were obtained to identify these organisms and a previous sampling using the most probable number technique to calculate how many colony-forming units (CFU) existed in the place. Cleaning was performed in limited sectors, using different techniques: hydrojet washing with controlled pressure, hydrojet washing, brushing with sodium hypochlorite 40% and with one neutral pH detergent. Once the wall was cleaned, it was rinsed with abundant water, allowed to dry during three days and each sector was subdivided in three. In one sector a hydro- repelling agent with a base of sylane in solvent was applied; in other, a product with a base of syloxanes and alcoxisyloxanes in solvents; whereas the third was left without treatment as control. New microbiological counts were performed after 1, 3, 6, 9 y 12 months. In general, low CFU numbers, except the sectors cleaned with hydrojet- washing and brushed with pH neutral detergent, whereas the hydro-repelling agents showed a protective effect, without great differences between the products.