Metallics Appearance Repeatability

We make a line of scientific instruments and are in the process of introducing a new line. We are having some issues with a gold metallic paint used on them, in terms of the repeatability of its appearance. Since we don’t do painting in-house, the parts in question are outsourced to a custom painting shop. When they come back, they are all over the map in color and gloss.

Q. We make a line of scientific instruments and are in the process of introducing a new line. We are having some issues with a gold metallic paint used on them, in terms of the repeatability of its appearance. Since we don’t do painting in-house, the parts in question are outsourced to a custom painting shop. When they come back, they are all over the map in color and gloss. It is almost as if they used different paints to paint different parts. The painting shop claims they are using the same batch of paint and applying it using a conventional air atomizing spray gun. I was wondering if you can give me some help in handling this problem. M.E.

A. Unlike conventionally pigmented paints that use spherical particles, metallics use pigments that are flat metal or mica platelets. To achieve the metallic appearance, the metal or mica platelets must lie flat in the film. Otherwise, the appearance can vary in color and gloss.

It is often difficult to reproduce the color and gloss of metallic paints applied by spraying because they are sensitive to a number of factors, including wetness of spray, percent solids, viscosity, solvent blend, solvent evaporation rate and paint temperature. Appearance could also vary from batch to batch. Since the parts in question were painted using the same batch of paint, the problem is due to the paint application procedure. Since you are outsourcing the parts in question, you must insist your supplier follow this strict application procedure:

1) Use the same spray gun to paint every part.
2) Use the same spray gun setting, fluid flow and atomizing pressure. If he is using an airless spray gun, he must use the same setting on those spray guns when painting every part.
3) Have the same painter paint every part.
4) If a thinner is used, he should use the same solvent blend to paint every part.
5) The paint should be at the same viscosity and temperature when painting every part.

If the parts are sprayed automatically, the supplier must use the same spray gun settings, distance to parts, line speed as well as all the aforementioned paint parameters to get consistency.