Dirt and dust may build up to the point that a mere rinsing with water from a hose will not remove them. A mild detergent and warm water will generally remove stubborn dirt and grime.
Mildew appears as dark spots or gray, fan shaped spots on the wood surface. Severely infested areas may appear uniformly gray or black. To remove a mild case of mildew, scrub with a mild cleanser or detergent. Next rinse with a household bleach to kill surviving spores. Lastly, rinse with water. When applying a new finish, be sure it contains a mildewcide. Note: Household bleach should never be mixed with detergent containing ammonia. Fumes can be fatal.
For severe mildew infestations, scrub with a stiff bristle brush using a solution of one cup of trisodium phosphate, one cup of liquid household bleach and one gallon of warm water. Rinse thoroughly. If necessary, follow with an application of 4 ounces of oxalic acid crystals dissolved in one gallon of warm water in a non-metallic container. Apply evenly with a soft brush. When wood dries, rinse with water. Caution: Oxalic acid is poisonous, but not dangerous it precautions are taken. Wear rubber gloves. Avoid contact with skin or eyes.
Nail stains are an unsightly problem that can be avoided by using stainless steel, aluminum or top quality, hot-dipped galvanized nails. The cleaning method described above for removing heavy mildew stains is suggested for nail stains. To help prevent recurrence, countersink the nails and swab the holes with a water repellent. When dry, fill the nail holes with a non-oily wood filler for natural finishes, or putty if the wood is to be painted.
Paint peeling, blistering and flaking occurs when moisture under a non-breathing film finish destroys the film’s adhesion to the wood. A properly installed vapor barrier is the recommended way to control this problem. Vapor barriers should be on the warm side of the wall. Problems also may be caused by faulty surface preparation, or the use of incompatible materials.