I've varnished a few masts. Generally, we have hung them by whatever is convenient at a height which is comfortable to work on both top and bottom; roughly upper stomach to lower chest height. Then work you way from one end to the other. You can be less concerned with drips, sags, holidays, etc. above about 10 or 12 feet above the deck. Nobody can see it anyway. From there on down, do a great job, the more coats the better.
I remember, a long long time ago, we had chartered a classic wooden ketch in the Virgins. Two masts, both wooden, the main about 110 feet tall. Both varnished beautifully. I asked the Captain who had told me he and his family did all the varnish work on the boat if he had any hints he could pass on to us neophytes. He said. "Yep, start at the top."