Air-abrasive methods have been used for the cleaning of outdoor metal sculpture for the past 11 years. The best known method among these is glass shot-bead peening. Other air-driven abrasives, such as walnut shells, offer an alternative which is gentler and different in action. Many of the fundamental ideas surrounding the various air-abrasive methods have not been rigorously examined. These include assertions that the methods do not remove any metal, that they passivate the surface by peening, that they reduce the corrosion rate, that it is necessary to remove all the corrosion products from the surface before applying a protective coating, and that the methods do not alter the aesthetic appearance of the surface. These assumptions will be examined both on theoretical and practical grounds using such methods as the scanning electron microscope, corrosion rate measurements, and a review of the literature on erosion by small, high-velocity particles, which is as yet poorly known to the conservation community. The authors illustrate the differences between abrasives and show that a fundamental understanding of the physical and chemical processes involved is necessary for their appropriate application.