From my days as a sail boat owner....
get your self a paint scraper that looks like an upside down 'U' in profile. It is sharp and you can drag it along the surface of the wood and all loose varnish will scrape off quite easily. This will also plane off a layer of wood to give you nice fresh material to work with. If you relieve the corners a bit with a file, the scraper will leave the metal alone. Always try scraping before you sand something wooden. Then get some marine product called Seikkens. It outlasts spar varnish by double in a salt water environment. Check online for wood finishing technics on boats and you will be away to the races.
