In order for a weapon to accept a finish, it must have several additional textures prepared. Those textures store important information about how the weapon is formed, which areas are paintable, and how easily the finish gets worn on different parts. The majority of the stock weapons can accept finishes.
A compositing system combines information from the finish with information from the weapon’s textures to create a believable application.
The finishes themselves are relatively simple. They control things like the color, pattern, and shininess of the finish.