Different weathering processes have been found at the Cueva del Raton, one of the hundreds of painted shelters located in the Sierra de San Francisco in Baja California, one of the most arid areas in Mexico. Although the paintings of nearby shelters are about 3000 years old, their state of preservation is quite surprising, considering the open-air conditions. There are, however, some conservation problems, essentially linked with the action of water or humidity. The presence of silica-rich layers associated with the rock art figures is discussed, since their role in the conservation of the site has been both beneficial and damaging. The presence of manganese oxide-rich layers is also mentioned. These layers do not cause a physical or chemical damage, but they do obliterate the figures they cover. A monitoring of the micro-climate conditions of the shelter is proposed, in order to fully understand the role of humidity and temperature in the weathering processes. -- SCMRE