This is spawned from a rather silly thread on a Woodworking forum on the Internet. Someone asked if linseed oil could be used on metal, particularly woodworking tools to prevent rust. The response was that linseed oil left a gummy film on the tools and they recommended all sorts of petroleum distillates to use instead.
Obviously some of the people responding have never learned to wipe off the excess before it dries. This will indeed form a gummy film and cause problems. The recommendations to use mineral oil [a laxative] or paraffin oil in one form or another to protect metals. If you are going to use petrochemicals on metal use something cheap like 30 weight motor oil, it will protect the metal with a non drying oil. People even recommend mineral oil for wooden food utensils, because it never dries it comes out in the food and is particularly harmful to wood which is a natural product and the synthetics are not. Then there is that laxative thing.
Linseed oil has been used as a metal finish and rust protector for centuries. Yes that is hundreds and hundreds of years. Linseed oil is used to quench hot wrought iron to give it a nice black finish. Linseed oil is also used to finish the iron by heating it up and dunking it in linseed oil. Linseed oil is a long bond double molecule, and while it sounds big it is actually very small and will penetrate into anything from iron and steel to woods and other porous materials and even smooth modern plastic laminates.
When done properly linseed oil will offer fine protection from rust to any ferrous metal. The key word is properly, that means wiping off all excess. That means there is no extra oil left on the surface, and this is where people end up with a gummy finish, they don’t get all of the oil. A thick coat of oil will dry from the outside in by forming a skin on the surface, preventing the oil underneath from drying, hence a sticky mess. A very thin coat will dry quickly and not have that problem.
Living in the dry arid West, rust is not that much a problem unless using green or partially dried wood, in which case the wood and the shavings will cause rust if left in contact with unprotected metal surfaces. Using wooden hand planes the only rust occurs on the irons so having some protection on the irons does help reduce the problem as does removing shavings completely before storing. Also sawing damp wood can cause problems especially with the swarf left in the teeth of the saws. My saws are all made from old, usually pitted blades that have been cleaned of surface rust, using a variety of methods. The blades are then heated up and treated with linseed oil, the excess wiped off and allowed to cure. The sharpening removes any oil from the teeth, so they can be vulnerable to rust.
The tools don’t gum up because they don’t have a thick film on the surface. The old iron and steel are protected from rust, except on their sharpened edges. Linseed oil was also used on firearms, on both the wood and metal parts for the very reason that if properly applied it offers protection to the wood as well as protecting the metal from rust. It is also the traditional finish, again when properly applied, to much of the iron hardware used on furniture and other wooden objects during the nineteenth century.
For those of you whom are of the same opinion, I apologize for being redundant, for everyone else, I believe that you are all entitled to my opinion.