DEAR TIM: We live in a shady wooded area. Our house is covered with natural cedar wood siding that constantly develops mildew stains. Every few years we apply an additional coat of oil based semi-transparent stain to make the house look fresh. What products are best to use to clean the siding and to seal it? How long will a good water repellent last? Susan P., Highland Park, IL

DEAR SUSAN: It is no wonder you have mildew problems on the siding. Millions of mold spores are in the air surrounding your house. They land on the siding and once there are delighted to find out that you have provided them with dinner. Many oil based house stains contain natural resins like linseed, vegetable, tung oils and animal fat. These water repellents happen to be food that mildew thrives on. The shady conditions slow natural evaporation and help to raise the overall humidity around the siding. These conditions are excellent for mildew growth.

The manufacturers of many exterior stains know this. They add chemical mildewcides and fungicides to the stains. But ultraviolet (UV) rays from sunlight and rain water break these chemicals down. Some stains contain very little of these products and within a short amount of time the mildew can start to grow.

In my opinion, the best semi-transparent stain to use is one that contains no natural oils. I prefer to use products that are made with synthetic resins. Mildew will not eat these products. It is still possible to get a small amount of mildew growth on the siding but the food source usually can be traced to airborne dust or aerosol saps and sugars that rain down from your trees during the active growing season.

Before you apply any water repellent to the siding, you must deep clean it to get the best results. Avoid cleaners that contain chlorine bleach. You can readily identify this by looking at the product label. The chemical name for chlorine bleach is sodium hypochlorite. This chemical can readily bleach the gorgeous natural color from your siding. It is also highly toxic to vegetation and your valuable trees. A neighbor of mine slowly killed one of her trees by using chlorine bleach to clean a patio each spring.

A safe deep cleaner for wood siding is oxygen bleach. It is a powder that mixes with water. It is non-toxic to vegetation and trees and will not remove any natural color from the siding. You simply keep the wood siding wet with the oxygen bleach solution for 15 minutes. It will remove the mildew and deep clean the siding. Lightly scrub the siding before rinsing with clear water from a garden hose. Avoid the temptation of using a pressure washer. These devices can force water into cracks and seams. This water gets behind the siding and can travel into your home. The intense pressure from these tools can quickly erode and scar soft woods like cedar and redwood.

If you buy a top quality water repellent, it can last up to five to seven years before it needs an additional coat. These same products can be used on wood decks but will not last as long. The orientation of the siding on your home helps extend the life of the water repellent. Rain water runs off the siding. When the sunlight does hit your house the intense mid-day UV rays glance off the siding. Because deck lumber lies flat, water readily soaks into the lumber and the UV rays at high noon blast the wood with a direct hit.

Price is a good barometer for judging water repellent quality. The synthetic resins contain very expensive raw materials. When you are doing comparative shopping, look for the highest priced product. It very likely is made with synthetic resins. I have had the best success finding these water repellents at specialty paint stores and some log cabin distributors. Oxygen bleach is readily available on the Internet at www.stainsolver.com. Be sure to buy oxygen bleach that has the highest concentration of the active ingredient - oxygen bleach. Many of the oxygen bleach products contain a significant amount of fillers that simply take up space in the container. Furthermore, many of the heavily advertised oxygen bleaches are made in the Far East, not in the USA!