A good place to begin is to define a metallic. A metallic finish is anything that looks like metal, or any finish that has any type of mirror-like reflective effect, sparkle, glitter, pearlescent, silver, bronze, gold or chrome finish. Metallics comprises a huge range of types, but they all exhibit some sort of color change with different viewing angles.
Metallics are often very attractive finishes, but they present real challenges for touching up as anyone manufacturing products with a metallic finish will attest. Why is it so difficult to make touch up for a metallic? Can’t you just put it under the scanner? We wish! Unfortunately no such software exists for formulating a color match of a metallic coating. There are spectrophotometers and software that can be used to QC metallic matches, but it requires a multi-angle spectrophotomer and software which is incredibly expensive and it will just provide a measure for how good the match is. It won’t tell you how to make the match.
Metallics coatings are made with a variety of pigments that include aluminum and bronze pastes, aluminium and bronze powders and a range of mica and synthetic particles. Moreover, all of these different pigments come in different colors, flake shapes and sizes which contribute to the effect.
If this doesn’t sound complicated enough, then you also need to consider the application method used. Think of a metallic coating as hundreds of thousands of tiny reflective metal plates. How these little plates are aligned will affect how the coating looks and behaves (a mirror reflecting light towards you is light and away from you is dark). As mentioned above, metallic coatings change in appearance as the viewing angle changes. A super technical term for this is “the flip flop". The flip flop is a simplistic way to describe the lighter/darker shift you see as you tilt the metallic up and down. This is the light reflecting off the metallic particles in the coating. The behavior of the flip flop is greatly affected by the coating type and application method. For example, the electrostatic charge used to apply a powder coating aligns all the particles in the same direction. This alignment wouldn't exist for a liquid touch up, so even if the touch up was the same exact paint it would look different depending on the viewing angle. The powder-coated metallic may get lighter when tilted up while the liquid paint would get darker, and vice versa, so it just isn’t possible to make them exactly the same. When matching metallics we always match to a fixed 45-degree viewing angle.
These are just a few of the complicating factors we face when developing a touch up for a metallic coating. Often it just isn’t possible to create an exact match with metallics - physics is working against us, but we do the best we can and have developed a process and techniques for achieving the best results. We are also constantly adding new materials to our pigment library which expands and improves our capabilities.
We start by evaluating the part for color, particle size and flash (how reflective it is) to select our starting pigments, and the sheen to select the starting base. From there it is an iterative process where we intermix bronze, aluminum and mic pigments as well as colorants and dyes to achieve the best mix. It’s a fairly scientific process that involves a lot of experience. We keep meticulous notes for each step so if we ever need to start over, we can start from and improve upon the last best formula.