DEAR TIM: I have an unusual problem. My house is covered with redwood beveled siding coated with a light solid colored stain. A few weeks ago dark brown streaks began to stain the siding. These stains appear to originate from behind the siding. I am also beginning to notice blue-black stains at each nailing location. Do you have a clue as to what is happening? What can be done to prevent this staining? I. L.

DEAR I. L.: I'm afraid I have some bad news for you. You appear to be the victim of a phenomena called extractive bleeding. It can happen with just about any wood siding, but it is especially noticeable with redwood and cedar sidings.

Redwood and cedar are forest products which contain naturally occurring chemical extractives. Woods such as redwood and cedar derive their durability and weather resistant characteristics from these chemicals. However, these chemicals can dissolve quite easily in water.

In your situation, water from several possible sources got behind the siding. This water could have been water vapor that condensed on the back of the siding after it traveled through the walls of your house. It could also have resulted from a leak in your roof. There are other possibilities as well.

The water then possibly traveled down the back of the siding and dissolved some of the extractive chemicals. When the water finally broke through a seam in the siding it ran down the siding, evaporated, and left the brown chemicals behind. This same thing can happen if water enters redwood from the surface. Water soaks into the wood, dissolves the chemicals and then is drawn back to the surface by wind action or sunlight. In any event, the result is the same.

The stains at each nail head are being caused by a chemical reaction between the extractive chemicals and the iron in the nails. Had the nails been hot dipped galvanized or stainless steel, you would not have had this problem. It may be in your best interest to replace the nails to prevent further staining. However, you can try to countersink the nails, swab the holes with a high quality water repellant, and when dry, fill the holes with an exterior wood filler.

The stains caused by the extractive chemicals and the nails can often be successfully removed. Mix one cup of trisodium phosphate, one cup of bleach and one gallon of water and wash the siding with this solution. Follow this with a solution of four ounces of oxalic acid crystals dissolved in one gallon of warm water. Be sure to use a plastic bucket to mix this solution. Wash the wood with this solution and let dry. When the solution dries, rinse with fresh water. Be careful! Oxalic acid is poisonous and can burn your skin and eyes. Wear rubber gloves and use goggles. Keep away from plants as well.

There is a way to virtually eliminate extractive bleeding stains. The trick is to seal completely each piece of exterior wood before it is installed. Just make sure the wood is completely dry before beginning the sealing process. In fact, the California Redwood Association recommends that this practice be followed with any exterior finishing system.

Should you decide to paint redwood in lieu of staining, this sealing process or back-priming is absolutely essential. The reason this sealing process is necessary is quite simple. By sealing the entire piece of wood, it becomes virtually impossible for water to enter the wood and dissolve the extractive chemicals. In the case of painted wood siding, the sealing process also prevents blistering of the paint. Blisters can only form if water has soaked into the wood. Be sure to seal the cut ends of each piece of wood as well. The carpenters may charge a little extra for this service, but it is well worth it. Remember, the end grain of a piece of wood is where water can most easily enter.