Exterior Varnish / Spar Urethane

Spar varnish and spar urethane are formulated to flex with the wood as it expands and contracts through seasonal changes, resist UV, and hold up to moisture. Standard interior poly will crack and peel outside in a single season. For exterior furniture, gates, doors, and anything exposed to weather, you need a product specifically rated for exterior use. The job isn't particularly difficult but it takes patience — multiple thin coats, sanding between coats, and annual or biannual maintenance to keep the film intact.

Sand to 120–150 grit. For wood that's been outside, clean thoroughly with wood cleaner and let dry completely — even several sunny days — before finishing. Wood must be below 15% moisture content. Fill checks and cracks with an exterior-rated wood filler.

• Thin the first coat 10–15% for better penetration.
• Apply with a natural bristle brush, working with the grain.
• Allow 24+ hours between coats (longer in cool weather).
• Sand with 220 between coats.
• Apply at least 3 coats, 4–5 for horizontal surfaces or high-moisture areas.
• Inspect annually and recoat before the finish begins to crack or peel — once it fails, full stripping is required.

• Using interior poly on exterior applications — it will fail within one season.
• Applying in direct sun — the finish dries too fast and won't level.
• Not sanding between coats — adhesion will be poor.
• Waiting until the finish looks bad to recoat — by then you're stripping, not recoating.