All About Spar Urethane (The Outdoor Finish)

Spar urethane is a popular wood finish that provides excellent protection for both indoor and outdoor projects. Here’s a comprehensive guide on spar urethane, including application techniques, differences from regular polyurethane, and answers to related questions.

How to Apply Spar Urethane

1. Surface Preparation:
   • Ensure the wood surface is clean, dry, and free of dust
   • Sand the surface with 120-150 grit sandpaper
   • Remove sanding dust with a vacuum or mineral spirits-dampened cloth
2. Application Methods:
   a) Brush Application:
   • Use a high-quality natural bristle brush and apply thin coats in one direction. Work quickly to maintain a wet edgeUse a dry brush to smooth from dry to wet areas perpendicularly
   b) Wipe-On Method:
   • Thin the spar urethane 50% with mineral spirits. Use a clean cotton rag as an applicator. Apply thin, smooth coats, overlapping each stroke
   c) Spray Application:
   • Hold the can 10-12 inches from the surface
   • Apply light mist coats in a steady back-and-forth motion
   • Slightly overlap each stroke
3. Drying and Recoating:
   • Allow 4 hours of drying time between coats
   • Sand lightly with fine-grit sandpaper (220 or higher) between coats
   • Apply a minimum of 3 coats, with 4-5 coats recommended for maximum protection

Differences Between Spar Urethane and Regular Polyurethane

1. Flexibility: Spar urethane is more flexible, expanding and contracting with wood movement
2. UV resistance: Spar urethane contains UV-resistant additives, making it suitable for outdoor use
3. Oil content: Oil-based spar urethane has a higher oil content, contributing to its flexibility
4. Intended use: Spar urethane is designed for exterior projects, while polyurethane is better for indoor use
5. Water resistance: Spar urethane offers superior water resistance
6. Durability: Polyurethane generally provides a harder finish, ideal for high-traffic areas
7. Chemical resistance: Polyurethane tends to be more resistant to chemicals and alcohol

Related Questions

1. How many coats of spar urethane should I apply?
   Apply at least 3 coats, with 4-5 coats recommended for maximum protection.
2. Can I use spar urethane on outdoor furniture?
   Yes, spar urethane is ideal for outdoor furniture, doors, windows, fences, railings, and boats (above the water line).
3. Is sanding between coats necessary?
   While some products claim it’s unnecessary, light sanding between coats is generally recommended for better adhesion.
4. How long should I wait before using the finished surface?
   Allow 24 hours after the final coat before light use, and wait 3 days before subjecting the surface to normal usage.
5. Can I apply spar urethane over stained wood?
   Yes, you can apply spar urethane over stained wood. Ensure the stain is completely dry before application.
6. Is spar urethane suitable for all outdoor projects?
   While excellent for many outdoor applications, spar urethane is not recommended for decks, high-traffic areas, siding, or rough-sawn surfaces.

Following these guidelines, you can achieve a beautiful and durable finish on your wood projects using spar urethane.