Boiled Linseed Oil does offer resistance to liquids but would not be suitable for this application. The toughest finishes are varnish, both natural and synthetic such as polyurethane. To find out if your finish is already waterproof, test with lacquer thinner. Put one drop on the finish in an inconspicuous location. If it softens the finish, then it is lacquer. If the finish stays hard, it's varnish. Either way, keep in mind that you will not be able to duplicate a factory finish at home.