Color, Appearance Inconsistency in Metallic Powders
Metallic powders available on the market can be divided into two broad categories: "bonded" and "dry-blended." In a bonded powder, the metallic flakes are attached (bonded) to conventional powder particles (those combined of resin, pigment, various additives, etc.) to form a rather homogenous mixture--a mixture in which all particles have similar specific gravity, size, dielectric properties, shape, etc. For the most part, these bonded metallic powders handle and apply just as regular, nonmetallic materials.
The difficulties described in the question are often observed with dry-blended metallic powders. Especially when they are applied manually. The difficulties arise from the nonhomogeneous nature of the mix. The differences between the components combining the dry-blended powder mixture are of a dual nature-aerodynamic and electrostatic.