A finish is vital for woodworking to protect your woodworking project from water damage. Some of the most commonly popular finishes on the market are varnishes and stains. A stain sinks deep into the wood to protect it from damage, while a varnish sits on the top of wood. Two of the most popular types of wood protection on the market are polyurethane varnish and tung oil stain.
True tung oil is derived by pressing the nut from the tung tree. It has been used for hundreds of years by the Chinese for finishing furniture and waterproofing ships. By contrast, Polyurethane is a polymer that was invented in 1937 by Otto Bayer as a replacement for rubber during WWII. It is used extensively in several media, including wood finishes.
Many of the products that are manufactured as tung oil on the market are not true tung oil. Instead they are a mixture of polyurethane and paint thinner that are manufactured under the name tung oil. Polyurethane/paint thinner mixtures that are manufactured as "tung oil" are distinguishable from true tung oil by their ingredient list. Tung oil that is not true tung oil is listed as having petroleum distillate added.
True tung oil soaks into the surface of wood. Once applied, it can take up to 48 hours to dry. It can take up to seven coats of tung oil to make a product truly waterproof, and true tung oil requires an additional coat yearly to maintain its finish.
Polyurethane varnishes sit on the surface of the wood. A coat of polyurethane can dry within 12 hours. It only takes two coats of polyurethane to be truly watertight.
Polyurethane must be brushed onto the wood with a paint sponge or brush, whereas tung oil may be wiped onto the wood using a cloth. Tung oil will bring out the natural grain of wood and does not coat the wood in a film the way that polyurethane does. This is why woodworkers who want to emphasize the grain of the wood choose tung oil. Using tung oil is often referred to as "making the wood pop."
Polyurethane is considered to be ideal for high-traffic areas such as wooden flooring. However, because of the thick patina that polyurethane leaves on surfaces, it is not considered to be ideal for intricate woodworking pieces such as scroll work on trim or furnishings. For projects such as these, tung oil is considered by some woodworkers to be a better finish because it soaks into wood.