I believe Dr. Gaddy mentioned in his article on rust bluing, that he used either motor oil, or boiled linseed oil. I don't believe he specified used or new, but why use dirty. I have used both and like the boiled linseed oil better.
I heat the barrels warm to the touch and apply the linseed oil wih a cotton cloth, sparingly. In a day or so it hardens to a nice finish that lasts quite a long time.
With the oil finish, you have to heat the barrels also, it lets the oil get into the pores to seal them against the elememts and then you wipe it off. Linseed oil does the same, but doesn't have to be wiped off.
