Removing Wood Stains
Mildew is a fungus that grows on the surface of wood or wood finishes. It does not hurt the structure of wood, however, its grey and sometimes black appearance is unsightly. High humidity levels promote the growth of mildew. To test for mildew, simply put a drop of bleach on a suspected spot. Do this in an out of the way location. If it is mildew, the mildew will disappear in a matter of minutes. To remove the rest of the mildew, scrub the area with detergent mixed with bleach. NEVER mix bleach with ammonia or detergents or soaps that contain ammonia. The combination of bleach and ammonia produces toxic gases which can be FATAL, especially in unventilated areas! After cleaning, rinse the washed area with a solution of bleach and water to kill any remaining mildew spores.
If you are a victim of extractive stains, they can be removed. All you need to do is to mix one cup of trisodium phosphate, one cup of bleach, and one gallon of water. Use this solution to wash the stained area. Then, find a plastic bucket or pail. Now, mix four ounces of oxalic acid crystals with one gallon of warm water. Note, do NOT use a metal bucket for this solution! The oxalic acid will react with the metal and cause massive problems!!! Wash the stained area with this solution and simply let it dry. Do not rinse it off until the solution is dry. Any remaining stains should come off when you rinse off the dried oxalic acid. Be sure to use goggles and gloves when using oxalic acid. It can hurt your skin and eyes. It can also hurt plants, so spread some plastic over them.
Cement & Mortar
The lime which is a part of virtually every concrete and mortar mix can cause stains on redwood which cannot be removed. Redwood which is splattered with these compounds develops black stains which are impossible to remove. Take whatever precautionary measures necessary to prevent splatter.