Technical Counselor Notes
This will be the first in an occasional series of Notes for consideration by EAA Builder and/or Restorers. I have recently had discussions with several members about the use of Zinc Chromate primer for current applications. I am disturbed by the apparent misunderstanding of the State of the Art in Aircraft fabrication practices.
To the point:
Advice is given, in good faith, that materials and practices which were widely used 50 years ago should continue to be applied to current day projects. Often, those old materials, and their applications, were the best available at the time, so text books and manuals were written expressing those truths. And, if there had been no innovation or progress since that time, when I and others of my era learned the craft, these texts and manuals would be valid today. However, we have had remarkable improvements in materials used in aircraft fabrication and these modern materials must be applied in the appropriate manner. Even if the current project is that of restoring or building an airplane of the early days design era, the materials will likely be of the type available today, and these must be used appropriately. In fact, some modern paint systems may prove fatal to the builder if instructions are ignored. Some composite matrix systems may lead to severe allergic reactions unless instructions for protection are adhered to.
So now we have some comments on Zinc Chromate Primer. I asked for suggestions from the Technical Staff at Polyfiber, being local, but of International reputation. I will repeat what is the essential guidance of the advice I was given.
Zinc Chromate is a fine anti-corrosive coating but has many drawbacks.
- Chromates of any sort have become the “Asbestos” of the paint industry. They are known carcinogens and are banned in most places. Many manufacturers have stopped making Zinc Chromate primer. There are equally effective substitutes for Chromates used in today’s primers, most notably, Dupont Inhibisil.
Zinc Chromate uses either a lacquer or enamel base. Lacquer and some alkyd enamels wrinkle terribly when they are overcoated with solvent based paints or cements. These WWII technology Zinc Chromates are fine as stand alone coatings to prevent corrosion. They don’t work well as paint primers, or as a base to attach fabric. Also, remember the health hazard noted above.
- Cements react with lacquer or enamel based zinc chromate primers. Today, aircraft fabric is almost always installed by cementing it to the structure. Obviously, it is not acceptable practice to cement to a primer that wrinkles and fails to adhere to the structure. It is also appropriate to state at this time that varnish on wood structures is attacked by the solvents in cements and has the same failure mode as zinc chromate primer has on metal.
Zinc Chromate has been almost completely replaced by two-part epoxy or polyurethane based primers in today’s aerospace industry. Epoxy primers are the most popular because they provide a tough surface protection coating that never wrinkles or washes away when being overcoated with a solvent based material. These epoxy primers include anti-corrosive agents that perform as well as chromates. For these reasons, while covering aircraft with fabric, epoxy primers are used exclusively. On wood, epoxy varnish is used to protect and provide a safe surface for cementing fabric.
If you already have a zinc chromate primed part, and you will later paint or cement to that part, an overcoating of the old primer (rather than stripping it) with epoxy primer will then protect the old primer from being attacked by the solvents and your attachment will be as strong as the bond of the old primer. The same applies for overcoating wood surfaces that have been coated with a varnish that would be attacked by solvents. You simply spray with epoxy primer so it encapsulates the old coatings and it does not react to solvents after that.
Of course, there are other areas of aircraft construction where corrosion prevention receives, or should receive, attention. On metal aircraft, a number of techniques have been in use for some time and these evolved into selected levels of sophistication. The most rigorous of these is Navy Specifications for their aircraft subjected to salt water environment. A future issue of Notes is expected to approach some measures you may feel suitable for your particular project,
Your comments and suggestions are welcome. Address to Letters to the Editor, WingNut.
Jim Pyle, EAA Technical Counselor, EAA Chapter One
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