The captain was kind enough to haul be up the mast three times today as I sanded, cleaned, and varnished the mast.
We started by lightly sanding with a 220 grit sandpaper to scuff up the existing coats of varnish. The sanding was just enough to make the varnish a little rough and white/grey from the exposed edges. It didn't take off any significant amount of varnish, and didn't get near the wood.
After sanding we cleaned up with a tack cloth. This little cheese cloth covered in sticky glue. The tack cloth annoyed us so greatly that we've already obtained a sponge that claims to function as a tack cloth. The sponge is twice ths cost of the cloth, but is reusable so it will pay for itself in on time.
Finally, I applied a thin coat of captain's varnish with a foam brush. The existing varnish was captain's so we didn't seriously consider alternatives, though our research indicated that it was suitable.
The whole job took about 6 hours, including breaks.