When you are cleaning anything made out of stone, certain care must be taken, depending on the type of stone that it is and the size of the statue. Outdoor garden statues and monuments will need special care. Custom figures (also known as figuritas personalizadas in the Spanish language) are prone to all the barrages of the elements, including fungus, lichens, mold, and mildew.
Without regular cleaning, several layers of grime can build up which will eventually end up chipping or cracking the rock. A good statue cleaner will break through these layers, nourishing the stone and helping with restoration. Be sure to first determine what type of stone that you are working with, however, because the cleaner will depend on the stone.
Softer stones, such as limestone, sandstone, or marble, will need a more gentle type of statue cleaner. However, hard stones such as granite are more durable and will withstand the elements better as well as harsher cleansers.
A good rule of thumb is to give outdoor statues a thorough cleaning every six months if you live in a harsh climate or one that has a high degree of humidity; and on an annual basis in other climates. This will keep them in good shape, keeping them clean year-round.