Ever since there has been tubular steel aircraft, the standard method of tube interior corrosion protection is boiled linseed oil. Not to say that there isn't a better method, however, only linseed oil has passed the test of time. As Fokker has found out, newer and synthetic is not always better. Operators of Fokker F50 have to X-ray inspect their engine mounts.
Why Linseed Oil?
Farmers and blacksmiths use it to protect tools from rust. The original paint was a combination of linseed oil, pigment, and Japan dryer. For many years, the standard corrosion inhibiting paint was "red lead". It was mixture of lead oxide and boiled linseed oil. Structural steel used to leave the fabrication plants coated with red lead. It was used on ship's hulls, bridges, and even on water tanks. The linseed oil gave "red lead" the ability to creep or wick into riveted seams or overlapping courses of steel plates.
The Menai Bridge of Thomas Telford opened in 1826 and was the longest span at the time. It is is held up by 16 wrought iron chains treated with linseed oil. These chains have never been replaced. Compare this to the rusting and failing bridges in the US today.
Why Boiled?
The oil is not actually boiled but heated so that any moisture is removed and the oil is sightly oxidized so that it dries faster.
Famous Bridges that used Linseed Oil
Williamsburg Bridge 85-year-old bridge the strands were originally coated with linseed oil and graphite.
Brooklyn Bridge
George Washington Bridge
Golden Gate Bridge 68% red lead paste in a linseed oil carrier
Menai Bridge
"The interior of the tubes and end-fittings of the engine mount frames have been preserved with a film of preservation oil. Premature degradation of this synthetic preservation oil is considered to be the cause of the corrosion. "
