Q. We recently had a deck added to our house. Even though the deck is built of pressure-treated wood, we have some concerns about whether additional protection is warranted. Would a sealant help preserve the wood, or would it work to counteract the pressure-treating?

If your advice is to apply a water sealant, should we do it immediately or wait? We have been told that the chemicals in pressure-treated wood require some time in the open air to stabilize.

A. We have had numerous questions concerning treatment of pressure-treated wood with sealers. Many people are confused about the durability of the product.

While pressure-treated wood does not rot, it can weather badly from continual exposure to rain if left unsealed. There are treatments that will help protect the wood and enhance the appearance.

Koppers Co. produces two such products: Rain Coat is a sealer designed for use on pressure-treated wood, and Deck Stain (which contains a water repellent) is formulated to tint and protect all types of wood used outdoors, including pressure-treated lumber.

Another product that will help an old deck look like new is Wolman's Deck Brightener. It is formulated to remove dirt, mildew and algae from pressure-treated and untreated wood.

Simply mix it with water and apply the solution to the deck with a sprayer, mop, brush or roller. Allow the solution to stand 10 minutes, then rinse the deck with water, and brush moisture off with a stiff-bristle broom.

When the deck has dried thoroughly, treat with a water repellent or stain such as the two mentioned above. Another brand name to look for is Thompson's, producer of several products designed to treat and protect wood.

If your pressure-treated wood is still new, wait three months before treatment. Newly treated wood needs a chance to dry out.

Q. I am having a problem with mildew on one wall in my home. It is particularly bad after a rain. The mildew travels up the wall. We had it professionally painted a year ago, which seemed to help the problem somewhat. However, the mildew is now getting worse.

I have checked with painters, hardware stores and other people for information on how to correct this problem. No one seems to be of help. Can you give me any advice?

A. First check the ground area around the foundation. Be sure the ground slopes away from the foundation and provides for drainage of rainwater away from the house.

Do not place plants close to the exterior walls. Watering required for plants will only make your problem worse.

If you have a crawl space underneath the house, inspect this area for moisture problems. One method of preventing excessive moisture from this area is to cover the ground with heavy plastic or roofing paper. Be sure to overlap the seams of the material you use to cover the ground. Use bricks to weight down the material along the overlapping seams, or use roofing cement..

Youalso can use insulation on the underside of the flooring to help prevent moisture and condensation. In addition, you can use a waterproof sealer applied to the interior side of the exterior walls. Apply a mildew-resistant paint over the sealer.

Adequate ventilation is a must. You can get window ventilators or even small vents that are installed in the side of the house.

In short, your job is to see that moist air can escape. If it has a way to get out, it won't condense on cool surfaces. Sometimes just opening windows a crack will solve the problem. An alternative is to use a dehumidifier.

Send inquiries to Here's How, Copley News Service, P.O. Box 190, San Diego, Calif. 92112-0190. Only questions of general interest can be answered in the column.