Professional rod builders use either two pack epoxy high build resin, or sometimes a UV curing system. The epoxy systems give a great finish and are the only type available for amateurs. Prior to epoxy arriving on the scene, all rods were built using traditional varnishes, similar to clear yacht varnishes. This air dries slowly, and needs several coats to give a decent finish, but it remains a good option for the amateur. It is also easier to remove rings and replace them when worn, if they have been varnished in place with traditional varnish.
Traditional varnishes are easy to apply so here is a quick guide.
Traditional Varnish . First let me reiterate, practice on some scrap material, partly for experience and partly to see what happens to your thread colour with the varnish. If your thread changes colour to any great extent, you need to seal the whippings with either dope, clear nail varnish, or PVA wood glue. Now to begin varnishing. Take a small quantity of an outdoor polyurethane wood or yacht varnish, and dilute with thinners until watery.