Why Does The Color Of The Body And Bumper Look Different?
Color Talk
Answers to your questions about PPG refinish color
I painted the body and the bumper at the same time but the color looks different - why is this?
M. Ridgely, Baltimore, MD.
There are a couple reasons why a color can look different when sprayed at the same time from the same can of paint onto plastic vs. a metal panel - the causes are related to some fundamental differences in the properties of the two substrates.
Unless specially treated, plastics are good electrical insulators, and can therefore build up a static charge when wiped down during preparation. If it is not discharged, the static can migrate into the paint film, and if the color contains a flake pigment it can cause these flakes to repel each other forcing them apart and altering their alignment which will cause a shift in the color (a darkening on the "face"). The best way to tackle this problem is to make sure any static charge is removed with an antistatic wipe before application of the color.
The second difference is the thermal conductivity of the substrates - if you have sprayed solvent onto a metal panel on a warm day, you may have noticed that the panel cools down significantly as the solvent evaporates. The solvent is pulling the heat out of the panel as it evaporates, which speeds up the drying process - this doesn't happen to any great extent with plastic as it is a good insulator. As a result of this effect, the same paint, at the same wet film thickness, may evaporate at a different rate on plastic than metal and, in the extreme; a big increase in the evaporation time will give a flake pigment longer to "float" in film and can darken the face slightly. This can be compensated for using the techniques discussed in the "Repaint Reporter" article "Vanishing Act" (Vol. 65 number 1).
Why is air presure important in color matching?
Air pressure is important in color matching because the very tool used to apply the paint (the spray gun)must have air to operate. A spray gun uses the available air to get the paint out of the gun cup and more importantly atomize (or break up) the paint droplets coming out of the fluid tip. There must be a balance between the amount of fluid flowing through the gun and the amount of air used to atomize it before it hits the panel. Having too much or too little atomization will affect the way the paint material acts when it lands on the panel.
With too little atomization, the paint remains in large droplets causing it to be too thick to allow the pigments and flakes to "align" evenly. Most often, the flakes will be near the bottom of the film and the pigments near the top. The result is a darker color shade and the missing "sparkle" of the flakes.
Automotive spray guns are designed by the manufacturer to be used with the controls wide open and at a specific pressure (PSI) coming into the gun. The design of the gun takes the amount of air coming in to create the proper amount of atomizing air at the cap. Changing these settings on the gun are not necessary if you use proper application techniques (gun speed, path, distance, mix ratios, etc.). If you vary any of those application techniques, you may need to adjust your spray gun to compensate to the correct fluid-to-air ratio.
PPG Color Marketing JANUARY 2006
