We were puzzled by the left side of the Curtiss P-40 Tomahawk — it sweats oil.
All of the seams and rivets show the telltale glossy sheen of some sort of greasy compound on the dull paint. No matter how much you clean, the oil comes back, even when the plane is simply parked.
The issue comes from the design of Curtiss’s oddly-placed oil filler (the gray cap at the top of the photo). When mechanics put oil into the fighter’s oil tank, a few drops inevitably fall between the canopy plastic and the side of the fuselage.
The oil makes its way down the inside of the plane’s structure and, well, keeps the skins and rivets of the P-40 “properly lubed.”