Hey Michael, the first idea that came to mind was using Owatrol Marine Strip, but I seem to remember something in the technical data about it not stripping polyurethane paints, so I’m guessing it might be the same for marine varnishes that are often made from polyurethane. Strippers can work wonders, but they’re messy and you need the right type.
    Obviously, as the mast is a cylinder that rules a scraper out of the equation. What about getting a piece of hard foam and cutting it to the shape of the mast, then binding to it a low low grit sandpaper over the shape using contact adhesive or epoxy. If you use the belt sander pads, they last a longer than standard sand paper a hell of a lot longer,

    If these all faile perhaps you could try the wooden boat forum –
    there’s a lot of experts on that page, they will have done it for sure! https://www.facebook.com/groups/45928256385/

    Hope that helps