Clear coat for pine tongue and groove ceiling

Spar urethane

If you want to spray a spar urethane (or really any type of clear coating), an HVLP sprayer (cup gun / compressor) is what you need to use. An airless sprayer is going to give you bubbles and runs that will ruin the finish. Bear in mind you will likely need to thin the product in order to spray effectively.

Are you looking for something with a gloss or more of a clear sealer? You can spray a stain/sealer with an airless rig.

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I'm wanting something that will help prevent mildew and make it easy to clean while keeping the natural color of the pine.

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A true spar varnish is an oil-based product, which means it is more likely to mildew.
There are water-borne spars now that offer easier clean-up. Minwax, Rustoleum, and Varathane all offer them. Since you want to spray, you may want to look at the Varathane product. It can be sprayed with an HVLP system. Not sure about other products. You can check their data pages for info.

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.