Clear coat for pine tongue and groove ceiling

Marine Grade Spar Varnish is what all the custom door builders I sell to say to use. Especially around salt water and wet humid climates. I don’t think Spar Urethane is a wrong answer either. Full disclosure I’m in the import hardwood industry not the finishing business.

There are many good options, but an easy to use product is Helmsman Spar Urethane in the oil modified waterborne version. Good enough for front doors, treated as a water based product and won’t yellow or give you mold issues that happen with true oil based products. You can get at Sherwin-Williams or Lowe’s. Marine grade varnish is awesome too but more difficult to work with and probably overkill for your project.