Wood oxidation is the result of years of exposure to the elements and causes unsightly markings on the wood's surface. The appearance of oxidation is more frequent on metal objects, which suffers from rusting. However, metal rusting negatively affects wood if attached to door or shingle. While it's only a matter of time before wood experiences some sort of weathering, homeowners are able to restore or prevent damage caused by oxidation.
Oxidation is the when a chemical element or compound comes into contact with oxygen. In chemical terms, oxidation is the result of atoms losing electrons. As a result, oxidation causes physical changes to objects such as wood and metal. Wood comes into contact with oxygen through a myriad of channels such as rainy weather, overexposure to sunlight and windy conditions. Oxidation occurs on building doors, garage doors, roofs and outdoor decks.
When wood constantly comes in contact with oxygen, the wood experiences weathering and, over time, deteriorates and loses its coloring. The result of oxidation on metals is rusting and the rusting spreads onto wooden doors with metal hinges. While it does not deteriorate the wood's structure, rust causes iron stains to appear on the door. Iron stains are brownish or gray streak-like markings, which are difficult to remove. The United States Department of Agriculture says the woods most susceptible to iron stains are oaks, cypresses, cedars and redwoods due to their large amount of tannin-like extractives. Iron stains also appear on wood if rust spreads from a metal cutting machine or nail.
While sunlight and the elements are responsible for wood oxidation, homeowners avoid iron stains by checking to see if oxidation occurs on attached metal objects. Pour a solution of sodium hydrogen fluoride or oxalic acid on the infected area of the wood's surface. If either solution removes the stain, then metal caused the iron stains on the wood. Another way to determine if metal caused the wood's stains is pouring bleach on the infected area. When bleach removes stains, mildew from water exposure is often the cause of the markings.
Bleaching and power washing are the primary methods for removing iron stains or other oxidation-caused markings on wooden walls, decks and roof shingles. Power washing requires machines spraying water at pressures of 1,000 to 1,500 psi. For more effective results, spray a layer of bleach over the markings before pressure washing. Texas A&M University's Texas Forest Service program suggests only repairing wooden roof shingles younger than 20 years, or 15 years in states near the Gulf of Mexico. Oil- and water-born preservatives are also available for spreading over weathered wood.