If you have cedar fence panels, you need to know that, with time, it naturally becomes gray or silver tone. If, by contrast, has a near-new or old cedar with black markings or spots, then you can clean your near sunspots, and brightness of wood.
Mold and mildew grow in cedar fence panels or other simply because they are developed in a plant environment. If you want to know difference between a speck of rust and tannin stain, apply a small drop of bleach. If stain disappears, then you have mold. Use a solution of water and bleach to remove dark discolorations, taking care not to use pure bleach because bleach timber completely.
Oil and sap are another cause of black discoloration cedar. Sap oozes to surface of wood, leaving a bright, dark in color, and is often sticky. When cedar is cut at sawmill, sometimes dripping oil on wood machinery. Use thinner to remove both sap stain and oil.
Cedar wood vanishes and takes on a gray appearance naturally with age. While you cannot restore its original color cedar fence panels, wood may lighten spraying with spray of acid. And if above solutions to remove oil, sap, mold, mildew and tannins not work, try this method using oxalic acid