Cleaning a Cedar Fence

If you have a cedar fence, you need to know that, over time, it will naturally turn to a gray or silver hue. If, on the other hand, you have a cedar fence—new or old—with black marks or stains, then you can clean your fence of the stains, and brighten the wood. While brightening it will never make the wood look new again, you can clean the fencing to improve its look.

There are four common causes of discoloration of a cedar fence, and they are:

• Natural fading and graying.
• Mildew, mold, and tannin stains.
• Oil and sap discolorations.
• Burns.

Black marks caused by mildew, mold, and tannins are the most common stains you'll find on your cedar fencing. Tannins are naturally-occurring plant chemicals that leech from the cedar wood. Since tannins are incompatible with metals and iron, any metal that comes in contact with cedar will leave black stains, including saw blades, nails, and even the metal bands used to strap lumber during transport. The redder the wood, the darker the black stains because the darker wood has more tannins.

Mildew and mold grow on cedar—or any other wood—simply because they thrive in a plant environment. If you want to tell the difference between a mold stain and a tannin stain, apply a small droplet of bleach. If the stain fades, then you have mold. Use a solution of water and bleach to remove dark discolorations, taking care not to use bleach full strength, as it will completely bleach the wood.

Oil and sap are another cause for black discolorations on cedar. Sap oozes to the surface of wood, leaving a shiny, dark discoloration, and is oftentimes sticky. When cedar is cut at the sawmill, sometimes oil drips onto the wood from the machinery. Use paint thinner to remove both sap and oil stains.

Cedar wood fades and takes on a gray look naturally with age. While you cannot restore cedar to its original color, you can lighten the wood by spraying it with an acid spray. And if the above remedies to remove oil, sap, mildew, mold, and tannins don't work, try this method using oxalic acid:

1. Wear protective gear. Acid is a corrosive and can destroy skin and tissue. Cover all metal fencing parts (except nails).
2. Mix four ounces of acid crystals with one gallon of warm water.
3. Wet the fencing with your water hose, and then use a plastic garden sprayer (with no metal parts) to spray the acid solution onto the wet wood.
4. Work in small sections and rub the solution into the fencing using a standard straw broom. Allow the solution to sit for thirty minutes, and then rinse entirely.

When you are finished, the wood will be a lighter color than before, and all of the black stains should be gone. If not, you can spot-clean tough stains and scrub and rinse again.