A coated metallic substrate including at least a first coating consisting of aluminum is provided. The first coating has a thickness between 1.0 and 4.5 m and is directly topped by a second coating based on zinc, such second coating having a thickness between 1.5 and 9.0 m. The thickness ratio of the first coating with respect to the second coating is between 0.2 and 1.2.

The present invention addresses the problem of providing: a top coating agent which forms a protective film demonstrating a beautiful silvery-white appearance, can be adjusted to suppress increases in the friction coefficient even when heat-treated at a high temperature, stabilizes sliding, is strong, and has excellent water resistance, corrosion resistance, and the like; and a fastening member having the protective film formed using said top coating agent. The present invention provides: a top coating agent that includes a polyoxyalkylene amine-based surfactant, a water-soluble modified organopolysiloxane, aluminum flake, and a diluent containing water; and a silvery-white fastening member having a protective film formed using said top coating agent.