Corogard

Corogard*—also spelled Corrogard, Coroguard, Coro-gard, etc—was required by the Navy on the forward facing edges of inlets, wings, and empennages of some airplanes beginning in the mid 1950s. It was a clear epoxy paint that provided an additional degree of erosion and corrosion protection to those surfaces. However, the Navy specified that it be "aluminized" by the addition of aluminum power.

It's not clear when this requirement was first implemented. Don Fogal found a reference to the requirement in Squadron/Signal, Navy Air Colors, Vol. 2, 1945-1985, which cited specification MIL-C-18263(Aer) dated 23 February 1955 that revised paint and markings, including the introduction of the new gray/white paint scheme.

"The leading edges of airfoils and frontal surfaces were to be painted in an approved rain-erosion resistant finish. At this time the approved finishes for metal surfaces were an aluminized color and those approved for glass fiber reinforced plastic assemblies varied from Natural Tan to Black. The erosion-resistant finish was to be applied in such a manner that it would extend from the first row of rivets or fasteners on the upper surface to the first row of rivets or fasteners on the lower surface."

The use of Corogard or a clear erosion-resistant coating on the leading edge of the wings and empennage of jets predates this document and was not necessarily limited to the first row of rivets. However, the width of the erosion-resistant application was almost certainly controlled by a paint and marking for each airplane type and therefore always the same. For example, this is the Vought drawing that defines it for the F8U wing.

Note that the Navy repair and overhaul facility finish instructions might differ.

The leading edges of Navy jets were originally painted with the overall color.

However, high speed flight tended to remove the paint from the leading edges of the wings and empennage. The original fix appears to have been to leave these surfaces unpainted as on this early F9F Panther.

 Also see http://tailspintopics.blogspot.com/2012/01/f9f-3-panther.html

Note that the amount unpainted surface on the Panther was subsequently increased.

There was likely some sort of corrosion protection provided on the leading edges, possibly a clear coat or post flight oiling. In any event, the use of the Corogard to protect the leading edges soon became standard. The practice was reportedly initiated at Grumman although I haven't been able to confirm that.

This is an example of the requirement for leading edge protection on the McDonnell F-4 (F4H) at a Navy Repair and Overhaul Facility:

This is a picture of the Sageburner F4H at the Smithsonian's Silver Hill storage facility. I'm not sure how soon the inlet was covered after the airplane was placed in storage by the Navy.

Since it was a relatively thick coat of clear epoxy mixed with aluminum powder, the perceived color and shininess varied with lighting and length of time since it had been applied. Some knowledgeable observers report it as being close to silver when new but that it weathered to a light gray. Another reported shade is close to aluminum lacquer paint. Another description is “semi-gloss medium metallic gray.”

Note the difference in the following two pictures between the leading edges of the wings and vertical fins.

It gets even more difficult to see in gray-scale pictures:

Unless the application is fresh and the lighting is right:

There are very complicated ways to replicate it, but I tend toward just spraying on matte silver.

It appears that at some point the Navy stopped using Corogard and left the leading edges natural metal, for example on the F-14. A subsequent change in the early 1970s was to use a clear polyurethane self-adhesive tape  developed by 3M on the leading edges, possibly only subsonic jets like the S-3 and A-6 initially. The tape would turn yellow over time from ultraviolet (UV) light exposure.

 
*3M filed for the wordmark "Corogard" in October 1951 and it was registered a year later. It is described as a "transparent protective coating material applied in liquid form ... said protective coating material drying after application to form a hard, tough transparent film." 3M allowed the trademark to expire in 1993.