Hi Ken sorry for the delay in replying I went on a fast trip to London to collect some parts for another back burner restoration project though not gun related.
I would stick to the well tried and tested formula for copal varnish and use Tung nut oil, I have made a form of copal varnish in very small amounts though quite a considerable time ago but according to my notes you use 2 parts Copal resin and I suggest artist quality to 3 parts oil. The oil will need to be extremely hot for the copal to dissolve as for adding the turpentine to the hot mixture I would give that a definite miss. Very hot oil and flammable turpentine could only be a recipe for a fire ball disaster, when the oil copal mix has cooled down it can become very thick then you add the turpentine to adjust its consistency to what you require.
