Spar (marine) varnish is best for exterior wood finishing, protection

Outdoor finishes all require maintenance due to moisture, heat, expansion, contraction and ultraviolet light. Moisture causes the wood to rot, and sunlight washes out its natural color.

You have some choices: exterior oil, Urethane, an epoxy sealer with an exterior varnish topcoat, or spar (marine) varnish.

Spar varnish is your best bet.

Exterior oil gives the least protection, and needs to be reapplied every year.

Urethane isn’t flexible during expansion and contraction from weather and moisture, and will crack and flake.

Epoxy sealers are the most durable but take longer to apply, and still need a top coat.

Spar (marine) varnish offers excellent protection from moisture and UV rays, and is flexible. It builds a protective layer around the wood and better durability than an oil finish. The term “Spar” comes from its use as a coating on spars of sailing ships.

Spar varnish is applied with a natural bristle brush in multiple coats. Every few years, sand the topcoat smooth, and apply a new coat of varnish.

Apply 3-5 coats, with the first couple diluted by 10-50% with mineral spirits or paint thinner for good penetration. The last coat or two is applied with a foam brush or quality natural bristle brush.