There are a number of affects that water can have on an outdoor metal sculpture.  Obviously, they are designed for an outdoor environment but things like untreated sprinkler water can cause accumulation finishes.  I have had clients with high calcium water have calcium deposits form on painted and stainless-steel surfaces.  This is often taken care of with a washing with CLR calcium remover with a soft cloth.  We have soaked painted samples in CLR for over a month and had no degredation of finish and we have concluded that this is a safe chemical to use.  The danger is in excessive rubbing – you could scratch the finish with the calcium as it is abrasive.  We suggest soaking it with a sprayer and rinsing until clean.