Woodworkers and builders often use varnish or polyurethane to protect wood from weather, acids and alkalies and the wear and tear of being handled or used. Before selecting a finish, it is important to understand the characteristics of each kind. It is also important to take into consideration whether the piece is an antique and is being restored to a former appearance, or whether the piece is new and has never been finished.
When time is of the essence, drying time of the wood finish becomes very important. Polyurethane is naturally fast drying, allowing multiple coats of this finish to be applied in one day. Varnish, an alkyd resin, on the other hand, tends to take longer and often needs 24 hours to dry thoroughly. For this reason, some manufacturers modify their varnish formulas to decrease drying time. These finishes are called modified alkyds. Urethane and silicone are common modifying agents, and are used to create a durable finish appropriate to commercial and industrial applications.
Spar varnish is a very durable finish. Originally developed for use on yacht spars, this type of varnish is able to withstand weather and UV light without breaking down. Therefore, the wood is protected. Polyurethane creates a waterproof finish, but it is not recommended for exterior use because it does not withstand UV light and soon breaks down in outdoor applications.
Appearance and Durability
Polyurethane is basically plastic. Polyurethane should not be applied over any other type of finish. Several coats of polyurethane may be applied on top of each other. However, unlike varnish, which tends to meld the underneath coat with the upper coat, polyurethane goes on as totally separate layers of finish. For this reason, polyurethane is subject to chipping. Polyurethane gives a hard finish. Varnish tends to have more of a patina, even in the high-gloss formulas. Varnish tends to wear well over a long period of time and can often be restored by sanding with 0000 steel wool and application of another coat of varnish.
Varnish is a light amber colour which tends to bring out the grain pattern in wooden pieces. Most varnish formulas include UV protection, so the colour of varnish tends to hold up over time. Spar varnish is one exception, as spar varnish tends to become very yellow with age. Polyurethane does not have UV protection and turns dark and yellow very quickly.