How to Sand Spar Urethane

Spar urethane is a clear waterproof wood finish. Originally formulated for use on boats, the spar is a part of the sail assembly, spar urethane is intended for exterior use in applications with high water concentrations. Modern craftsmen use spar urethane in interior and exterior for its clear finish and durability. It can be sanded as part of the finishing process.

Things You'll Need

• Spar urethane varnish
• Paint brush
• Paint thinner
• Cleaning rags
• Sandpaper (300 grit or finer)

Instructions

Properly Sand Spar Urethane

1. Sand the surface smooth before the application of the spar urethane. The smoother the wood, the better the resulting finish.

2. Apply the spar urethane. Spar urethane can be thinned with paint thinner or mineral spirits. A thinner mix may result in a smoother finish. If thin enough the spar urethane will level itself and erase brush or rag marks. Multiple applications of the spar urethane may be required.

3. Sand the spar varnish between each application of the spar urethane. Allow the finish to dry and sand lightly with the finest sandpaper available (320 grit). Only sand with the grain of the wood. Do not over-sand. The idea is to smooth the previous coat of varnish, not sand it away.

4. Apply the last coat of spar urethane with a lint-free rag. The wood is not sanded after the final application of the spar urethane.

Tips & Warnings

• Spar Urethane can be applied with a brush, rag or a high-volume, low pressure spray gun. Multiple coats of the varnish provide extra protection from weather and water.

• Spar urethane finishes can be refinished to improve the look of the finish and to improve its waterproof qualities. The finish is sanded, lightly in the direction of the grain, before the new coat of spar urethane is applied. This finish can be sanded and another coat of spar urethane applied if necessary.

• Make sure the surface is dry and clear of paint or wax. Stir the spar urethane thoroughly before applying. Expect each coat of varnish to dry in about six hours in conditions of about 75 degrees and 50 percent relative humidity.

• Spar urethane is not recommended for floors. The finish can become too smooth and become slippery. Spar urethane is also not recommended for fences or decks. Other forms of water proofing materials work best in those applications.

• While spar urethane is a clear finish the varnish should be tested in an out-of-sight area. Discoloring may occur if the urethane is applied over a stained surface.

References

• The Wood Whisperer: A Better Way to Apply Spar Urethane
• Minwax: Oil-based Clear Protective Finishes