re: Most Tung oil directions get this wrong

If you want to know all about linseed oil refining, use in oil paints, etc., Tad Spurgeon’s book can’t be beat. Here’s a link directly to the book. It also covers tung oil and walnut oil, though mostly dismisses them, because Tad’s methods will give a linseed oil which yellows very little (which is the primary reason for artists look at things other than linseed oil).

If you just want to know about refining linseed oil (he gives six different methods, none of which involve Japan driers), there’s a 16 page excerpt which is more digestible.