Posted by David Sherman on Saturday, January 22, 2005 at 2:51PM :
In Reply to: linseed oil undercoating posted by KBishop on Saturday, January 22, 2005 at 7:08AM :
Linseed oil is the oil that makes oily rags spontaneously combust. The keys to spontaneous combustion of oily rags are the same as with a pile of manure and straw or fresh coal or wet sawdust -- a big enough tight enough pile to accumulate some heat, but a loose enough pile to let oxygen into it. The safest thing is to throw the oily rag in the wood stove as soon as you're done with it. One rag by itself is not likely to catch fire, but a pile of them in the corner has burnt down many shops. Putting oily rags in an airtight container will prevent combustion and will keep them from drying out so you can keep using them. If it's summer and I'm done with a rag or paper towl with linseed oil on it, I'll leave it in the middle of the floor or on top of some large metal thing until it's dry, so that even if it catches fire it won't spread. Then it's safe to throw in the garbage.
Most old paints and varnishes had linseed oil in them and some still do. Any oil finish containing linseed oil (such as "Danish oil") will have a warning on the side about spontaneous combustion. You always want boiled linseed oil, since the raw oil stays gummy and soft for a long time. Boiling makes it dry reasonably fast. Raw linseed oil is basically the same as flaxseed oil sold for food.
The only time I've used it on metal was on some nice old axes, chisels, drawknives, etc where I wanted a little more protection than just wiping them down with motor oil, but didn't want any kind of varnish or enamel that would flake off. It seems to work well for that.