The green color you see on aged outdoor bronzes is looked upon by some as being an attractive patina.  What this color indicates, however, is the chemical change (that is, corrosion) of the bronze metal as it is exposed to acid rain.  Unlike a pure copper roof which will form a layer of corrosion byproducts which acts as a protection mechanism if left undisturbed, bronze sculptures suffer a substantial loss of surface material.  As the years go by and exposure continues, enough material is lost to make details of the sculpture disappear and appear nondescript. Chemical repatination restores the artist's original color and reclaims sculptural details.  A protective coating will prevent future corrosion.
						(Brush mouse across Before photo to see After photo)