Anyone who has ever read anything on the subject of painting and camouflage of US aircraft of the World War II era must have come across the name Zinc Chromate. Do you know what Zinc Chromate is? Understanding the nature of this coating it is an essential starting point for the discussion on anything concerning color coatings and protective finishes of US aircraft of the time.
What is Zinc Chromate
Back in the 1940s as well as in the paint industry of today, the term Zinc Chromate does not refer to a paint color, but rather a protective coating.
Zinc Chromate is a corrosion resistant agent that is added to certain coatings. Even today, chromate finishes including Zinc Chromate provide superior corrosion resistance. Additionally, Zinc Chromate is highly toxic thus protecting the surface from proliferation of organic matter.
In the aircraft industry of the 1940s, Zinc Chromate was used as an anti-corrosive barrier primer; it could be described as a sort of painted-on galvanizing. It has been developed by Ford Motor Company by the late 1920s, subsequently adopted in commercial aviation and later by the US Military. Official USAAC notes mention successful application of Zinc Chromate primer starting from 1933, but it has not been adopted as standard until 1936.
Because Zinc Chromate is all about corrosion protection, the precise coloring of it is and has not been considered as important as the chemical composition. In the official notes of the period, the name Zinc Chromate is often accompanied by the name of particular manufacturer, thus mentioning Ford Zinc Chromate, DuPont Zinc Chromate or Berry Brothers Zinc Chromate. This means that the actual color of Zinc Chromate coating may have varied from batch to batch or manufacturer to manufacturer without it being viewed as an issue.
The ‘native’ tone of zinc chromate crystalline salt is a bright greenish-yellow. When put into a vehicle with binders to make paint, this color would be the raw result.
Such raw Zinc Chromate primer would also give a semi-translucent coating, not very opaque like a pigmented paint or lacquer. This property becomes especially interesting when we consider that aircraft factory instructions often called for just one protective coat of primer. As a consequence, the color of the underlying surface might have a significant effect on the final appearance. For example, raw Zinc Chromate applied on the white background would look yellow, while applied to bare metal aluminum it would look more like apple green.
Similarly, any pigment might be added to the raw paint mixture to go with the Zinc Chromate, thereby modifying the color. Some of today’s mixtures use iron oxide — giving that rusty red appearance you can often see on prefabricated steel beams in highway and building construction.
So what does all this mean? Perhaps no more than there hasn’t ever been any specification in the industry for a Zinc Chromate color. This in turn caused alternative designations to pop up in the literature that attempted to describe the color value of the Zinc Chromate finish – Zinc Chromate Yellow and Zinc Chromate Green being the prime examples. These will be described below.
In US Aircraft industry, Zinc Chromate was in widespread use already at the outbreak of World War II. In comparison, Germany and other axis powers didn’t use it at all, relying on lacquer-based protective coatings – one reason why we never saw any Luftwaffe aircraft in bare-metal finish! The British adopted Zinc Chromate in their aircraft production starting with Martin-Baker M.B.5 of 1945, several years after the Americans.
Zinc Chromate Yellow
In US aircraft use in the 1930s to 1940s, the Zinc Chromate primer was frequently used in the raw mixture yellow tone. This is sometimes referred to as Zinc Chromate Yellow. Like stated above, there is no definitive color pattern as this may have varied between manufacturers and batches of these primers.
In the immediate pre-war and early war period, the raw yellow Zinc Chromate primer seems to have been dominating.
The raw Zinc Chromate primer was yellow in tone with just a hint of green, as can be seen here. The photograph shows working on the horizontal stabilizer for a Vultee Vengeance dive bomber at Vultee factory in Nashville, Tennessee.
Zinc Chromate Greens
Sometimes, Zinc Chromate was mixed with Lamp Black paste to give a bit more UV resistance (Zinc Chromate is very sensitive to photolitic reactions) and more durability in high wear areas.
Mixing with black gave greener tones, which, depending on the amount of black added could run from apple greens to medium olive greens.
There were many variations in Zinc Chromate Green. Originally, manufacturers were expected to mix raw Zinc Chromate, black enamel and aluminium paste or powder. Several blacks and greys could substitute for the black enamel, and a shortage of aluminium powder/paste caused a reformulation without it in 1942.
Some aircraft manufacturers ordered pre-mixed Zinc Chromate Green (Curtiss Cockpit Green, ordered from Berry Brothers, being an example of this).
There is evidence that such variety of shades occurred in the manufacturing practice of US aircraft factories. Where sufficient color evidence is available, it is possible to find all three colors used on the same aircraft – for example, the yellowish raw color in the wheel wells, the apple green tones in the gun bays, and the darker green in the cockpit.
A perfect example of Zinc Chromate Green can be seen here. This photo of the internal wing structure on Douglas A-20 bomber. The ribs have been covered with mixed (tinted) Zinc Chromate primer that we will refer to as Zinc Chromate Green.
Untinted and Tinted Primer
US Erection & Maintenance instructions of the period often refer to "untinted" and "tinted" primer to describe raw Zinc Chromate primer and the same primer tinted with black. While black was the intended additional pigment, the instructions did not specify the formulation of colors. Therefore it is not unlikely that manufacturers felt free to pick substitute pigments when needed. A Navy memo from 1942 goes even further and recommends using Indian Red, lamp black, or any other suitable indicator to use with a second coat of Zinc Chromate primer to distinguish between single- and double-coated surfaces. In the light of this memo, Vought’s Salmon pink was also simply tinted primer.
Salmon was a pale pink-colored chromate primer used by Vought in production of the F4U Corsair. It was produced by mixing Indian Red pigment with raw Zinc Chromate primer.
The actual tone was reddish orange.