You polish a metal item until it is reflective enough to be used as a mirror. This does not prevent the item from later damage or corrosion that would ruin the mirrored surface. The spell can be cast only on a metal item with a fairly smooth and contiguous surface, such as a breastplate, a helm, a shield, or a sword, but not a mace, chainmail, or scale mail.
Pathfinder Roleplaying Game Advanced Class Guide © 2014, Paizo Inc.; Authors: Dennis Baker, Ross Byers, Jesse Benner, Savannah Broadway, Jason Bulmahn, Jim Groves, Tim Hitchcock, Tracy Hurley, Jonathan H. Keith, Will McCardell, Dale C. McCoy, Jr., Tom Phillips, Stephen Radney-MacFarland, Thomas M. Reid, Sean K Reynolds, Tork Shaw, Owen K.C. Stephens, and Russ Taylor.