The bottle says Alkali Refined Linseed Oil purchased from a fine arts store and doesn't mentioned boiled so that may be the problem. If I were to substitute tung oil for linseed oil do I need to pay attention to how the tung oil is processed? The spar varnish I used is Old Masters exterior oil based spar-marine-varnish, so should be fine there. I mixed my Japan dryer as per instructions on the bottle which was 28ml per 4 litre which reduced down to 1/4 tsp per 4 tsp of solution.

Nothing is foolproof to a sufficiently talented fool.