I'm considering a shou sugi ban (Japanese burned wood) cladding on the building I'm currently working on. I've done a quick test with a few small pieces of the board I'm thinking of using and the result looks quite good, but it transfers black marks to anything it touches, even after quite extensive cleaning. Does this problem go away by itself, or do I need to apply some kind of treatment to prevent it transferring, and if so what would be appropriate? I've seen suggestions of tung oil, but that seems prohibitively expensive for covering a large area -- would linseed oil be appropriate?
Charcoal (if not burned) is forever, more or less (ask an archeologist if you doubt that.) As such I'd have to doubt that cleaning which does not go so far as to remove the charred surface will help, nor will time and weather do much.
Linseed oil would be a reasonable approach for a building exterior, though I'd certainly test that before proceeding.