Our dwelling, built in 1974, has an exterior of cedar boards. It was stained at least once, but needs to be redone. It is particularly bad on the sunny side of the house.

Can we stain it again with a stain to cover water marks and bring everything to a more uniform appearance? Is it worthwhile, after restaining, to cover the new stain with something colorless, such as Thompson's Water Seal, for longer life and weather/waterproofing, ? -- J.W.F. If the siding is badly discolored and water-stained, it would be best if you bleach the surface before applying a new coat of stain. Use a weak solution of oxalic acid, available at paint or hardware stores. Use with caution, wearing goggles, rubber gloves and old clothing. Brush on the exterior surface. Follow with a thorough rinsing with clear water.

When the siding is dry, apply two coats of a shingle stain. On the average, restaining is necessary every five or six years.

A penetrating clear sealer as a final coat will help protect the surface and repel water. Stay away from varnishes and other coatings that form a film on the surface. Check with your local paint dealer for recommended products. I would like to know how to clean my slate floor. It has some wax on it and the grout is quite messy and dirty looking. -- E.C. Slate is practically immune to all common chemicals and any of the usual cleaners that may be used.

Ammonia in the water may be used when needed, but strong acids and strong alkali may attack the grout.

Sand-rubbed slate, which is relatively smooth, may be scoured when necessary with steel wool or an abrasive pad.

Cleft slate, which is the natural surface after the slate has been split, is less easily scoured because of the textured surface. It is best scoured with a stiff brush.

After cleaning, you may find that the grout is not much improved. You may want to remove the top surface layer of the grout (about 1/4 to 1/2 inch) and regrout with a new layer.

A coat of good floor sealer (colorless on light slate) will improve the appearance and make maintenance easier. The sealer will also protect the grout.

Wax, either the emulsion type or the solvent kind, may be used on slate, though too much gloss detracts from the natural soft effect that makes slate desirable. In one of our closets, which has two outside walls, mold develops each winter. I close the door to the closet each night to prevent cold air from entering the room, but open the door each day to circulate the air.

The house has a central humidifier. Is there a treatment one can apply to prevent the mold from growing. -- D.W. You are right to try to open the doors as much as possible to provide for increased air circulation.

I suggest that you remove all articles from the closet and take clothing outdoors to brush away all mildew spores, or vacuum each item. Let clothing air for several hours. Wash all washables in hot water and an appropriate bleach; dry thoroughly. Send nonwashables to the cleaners. Shoes, belts, boxes, and the like should be brushed or vacuumed.

Next, tackle the mold inside the closet. The easiest way is with a vacuum cleaner. Vacuum the walls several times until all signs of mold are gone. Throw away the disposable bag and clean the attachments. You may also scrub the closet with a mild alkaline solution, such as trisodium phosphate and warm water. Be sure to test the solution first on an inconspicuous section of the wall to be sure it won't damage the surface. If it does, add more warm water and test again. If the walls are durable and water-resistant, you could use a solution of warm water and household bleach (50-50 mixture) to wash down the walls and remove remaining mildew spores.

When the closet is mold-free, sprinkle paradicholorbenzene moth crystals around to help absorb moisture and deter mold growth. Bags of para crystals can be attached to hangers, but be careful, as the crystals may damage plastic buttons and ornaments. You can also paint the inside of the closet walls with mildew-resistant paint.

To help prevent mildew in the future, you need to dehumidify the area (do not use a dehumidifier). A plain light bulb left burning is effective. There are also low-voltage electric rods that can be installed in the closet to prevent moisture buildup. Frequent airing of the area is also helpful. 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.