Stone statues are a good feature to have in any garden and apart from adding to the garden decor they require little or no maintenance. Hail, rain or snow will have little impact on a stone statue which is resilient to all types or precipitation including acid rain. The contamination most likely to be encountered by statues and other permanent garden features comes from animals and birds.
A good scrubbing brush is a necessity if the statue has a lot of bird droppings and warm soapy water mixed with vinegar does a good job in removing this type of dirt. If the statue is tall you will be using a steps or chair to reach the top (and usually most dirty) section, so great care must be taken as the standing surface will most likely become slippery from the soapy liquid. Garden statues can be restored to the as new state even after years of neglect and thereafter a regular soap and water washing, as described, will keep them in perfect condition.
Many of us choose to use chemicals to clean stonework pieces but all it really requires is plenty of old fashioned elbow grease. There is really no substitute for hard work and I done believe that chemical cleaning is as effective. I also fear that some of the chemical, if not properly handled may leak into the soil or water table so I avoid them altogether. I would rather have a slightly grubby statue than a pristine clean one that had contributed to a bad environmental effect as a result of its cleaning. I clean my garden statues twice per year, once in the Spring and then again in the Fall. I also clear away debris at the foot (or skirt) of the statue at these times especially if a mounting plinth is not used.