Tung oil has been used for centuries as a finish for wood and for many other purposes.The Chinese used tung oil to waterproof masts and sails for their junks. The U.S. military used tung oil to coat military parts during World War II. In addition to being water resistant, tung oil resists many alkalis and acids as well. Tung oil remains flexible over time and accommodates the continuous expansion and contraction of wood surfaces. It also maintains its color while other oils like linseed oil darken over time.
Pure Polymerized Tung Oil
Pure polymerized tung oil is the best kind for oak refinishing. Wod looses moisture over time; oils are used to restore wood to its natural appearance. As tung oil dries on a wood surface the molecules bind together, forming a tight bond that is waterproof and resistant to many chemicals. This bonding process also makes the surface flexible and resistant to wear and tear. Polymerization is a cooking process that alters the molecular structure of tung oil, intensifying its natural bonding tendency.
Ultraviolet radiation causes photo degradation, a form of damage to either the coating or the wood fibers. The initial damage is fairly minimal, but with continued UV exposure and compounded by moisture, the process soon causes extensive damage to the coating and then to the wood itself.
Preparation of the Wood
The wood surface needs to be cleaned of all foreign substances (lint, dirt, grease, wax, oil, old paint or varnish). New wood surfaces need to be sanded with a fine grade sandpaper. This allows maximum penetration of the tung oil into the wood pores.
Applying the Tung Oil
When applying the tung oil the temperature should be between 55 and 75 degrees F and the humidity should be less than 65 percent or you will have difficulty applying the finish and the finish will need more time for cure. Apply the first coat liberally with a paintbrush. Allow the application to soak in for 10 to 15 minutes then wipe the entire surface with a clean rag or cheese cloth. Allow the first coat to cure for six to eight hours. Let the coating dry; apply the next layer using the same procedure but with less finish. Then repeat the process for four more layers to get strong protection and a nice looking finish. To avoid streaking and flashing (dull spots in the finish from moisture trapped between the layers due to insufficient curing time) perform the cure test. This consists of pushing your fingertips against the grain of the wood over the various parts of the surface to be refinished; if there is any grab or friction, allow more time for curing. Repeat the test every couple of hours until your fingers slip easily across the surface.
The maintenance schedule will depend on the exposure to UV light. High exposure to sunlight, moisture or high temperatures will require annual maintenance. For more moderate exposure the maintenance will be needed every three years. Cracks in the finish are usually fine microscopic lines that are easily filled by applying another coat of tung oil. There is really no limit to the number of coats that can be applied. Simply apply another coat of tung oil when maintenance is due.