Color-Matching Problem

Our customer is rejecting parts because it says the flat black paint used on them is not black enough. How do we quantify the problem? Would a colorimeter be useful for inspection?

Ask an Expert From: 10/4/2013 Products Finishing, ,

Q. Our customer is rejecting parts because it says the flat black paint (MIL-PRF-22750, Fed. Std. 595 color 37038) used on them is not black enough. Color is very critical because our parts stack up with other parts, and the stack-up is visible to the final customer. We have the official color chip but are having difficulty judging acceptability.

The painted parts we receive look black enough when viewed straight on but can appear gray/black when viewed from an angle. Our supplier is using paint certified to the MIL-SPEC. The painted surface of problem parts tends to be rough to the touch and marks very easily. The roughness abrades off skin when touched with bare hands, or abrades off fibers and particles when wiped with a cloth or swab.

We are having difficulty rejecting parts that will not satisfy our customer because we don’t know how to quantify the problem. Would a colorimeter be useful for inspection? Do you have any recommendations for how to inspect and for how to correct the situation? —M.D.

A. Color perception, like beauty, lies in the eye of the beholder. It is a complicated matter. Simply stated, color perception depends on a number of factors, including the illumination, the background, the age and condition of the eye of the viewer, and the viewing angle. Even under the same illumination, background and viewing angle, it is possible that 12 different viewers will give 12 different description of the same color sample. It is amazing how many different color descriptions you can get from the same sample.

When inspecting colors visually or instrumentally, the paint film must be viewed at a 90-degree angle so that the viewer is observing reflected and not incident light. This will enable him to exclude specular reflection. This is especially true of ultra-flat finishes like those you seem to be applying. Furthermore, it sounds like your painted surface is as rough as an emery board. Perhaps you have an application issue that is contributing to your problem. I suggest you check the gloss first to make sure you are in the proper range.

Owing to the aforementioned facts, I believe you need the help of a color expert. You can’t make the color determination by committee, however; you must assign one person to be the color expert. It is important to have that person inspect each paint sample.

I suggest you contact suppliers of color and gloss meters and related equipment for a solution. You can start by going to pfonline.com and clicking on Suppliers, then Painting and then Testing Equipment, color or gloss.

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