Spar urethane and outdoor clear finishes
Spar urethane is the name makers give their clear finishes for outdoor wood. Minwax says Helmsman contains UV blockers to reduce graying and fading, and recommends three coats outdoors or on bare wood. General Finishes says Exterior 450 has built-in UV absorbers; it is water-based, where the other two are oil-based.
Across the three makers the recoat time is a median of 4 hours, from 2 hours for the water-based Exterior 450 to 8 hours for Varathane’s oil-based Spar Urethane, which Rust-Oleum rates for light use after 24 hours and full use after 3 days.
Outdoor clear finishes, by their makers
Outdoor clear finishes: recoat across makers
4 h
span 2–8 h
Median of 3 sources
Outdoor clear finishes: coverage across makers
570 sq ft/gal
span 380–760 sq ft/gal, per coat
Median of 3 sources
| Product | Touch-dry | Recoat | Use | Full cure | Coverage | Thinning and notes |
|---|---|---|---|---|---|---|
| Minwax Helmsman Spar Urethane One source: Minwax (Sherwin-Williams), the maker of that product | — | 4 h | 24 hnormal use | — | 125 sq ft per quart | UV blockers; 3 coats outdoors or on bare wood. |
| Varathane Oil-Based Spar Urethane One source: Rust-Oleum (Varathane), the maker of that product | 2 h | 8 h | 24 hlight use (full use after 3 days) | — | 380–760 sq ft per gallon | Thinning: Do not thin.Conditions: 70 °F, 50 % RH; apply at 55–90 °F.Handle after 4–6 hours; sand lightly if a coat has dried more than 12 hours. Rust-Oleum names railings, fences, patio furniture, house and trim, and not decks, high-traffic areas, siding or rough-sawn wood. |
| General Finishes Exterior 450 One source: General Finishes, the maker of that product | 30 min | 2 h | — | 21 days | up to 600 sq ft per gallon | Thinning: Water, GF Extender or GF Accelerator.Conditions: 70 °F, 50 % RH.Water-based acrylic/urethane blend with UV absorbers. |
Work it out for your project
Times are the makers' own at the temperature and humidity they name, and several of them say cooler, damper conditions slow their finishes. For how much to buy, the finish coverage calculator uses these same coverage figures. Every finish side by side: the finish dry time chart. What the federal rules say about finishes and food: food-contact finishes.
Elsewhere in finishing: shellac and the shellac cut calculator; for repairs before the finish, wood filler vs wood putty; and for the glue-up, wood glue types.