Q: We have a block wall surrounding our beach-area home. Pranksters have marked the wall with graffiti.

I am not sure about all of the substances used to mark the wall, but some of it was done with spray paint. We have been unable to remove these unsightly marks. Do you have any suggestions on how to clean these walls? -- M.F.

A: You will probably have to paint the wall, and it will be important to use a good primer so the graffiti will not bleed through.

A product often used by contractors is KILZ, manufactured by Masterchem Industries (P.O. Box 368, Antonia, Mo. 63012). This quick-dry primer-sealer has exceptional hiding power for covering stains -- even smoke stains, which are difficult to keep from bleeding through.

Cover the primer with a final coat of paint. Check with your paint dealer for products recommended for use on masonry surfaces.

Q: We installed dark brown tile on the floor of a covered patio and used a similar deep shade of grout. The grout has turned a blotchy gray and we are not happy with the overall look. Is there anything we can do to restore the darker shade? -- O.W.

A: The source of your problem might be efflorescence, which is caused by moisture seeping up through the floor and depositing salts on the surface of the mortar.

This can be corrected by scrubbing the mortar with a solution of muriatic acid (mix one part acid with nine parts water) using a stiff-bristle brush (not wire).

Wear protective clothing, eye goggles and rubber gloves. Pour the acid into the water, not the other way. Be careful to avoid splashing. Rinse the surface with plenty of clear water after the acid application.

If this restores the color, treat the mortar with a sealer recommended by your tile dealer.

You also need to correct the source of the moisture. Check drainage around the patio. The ground should slope away from the area. Watering plants can also be a source of your problem if they are close by.

It also could be that the mortar is fading and efflorescence is not the problem. There are stains available that can darken the grout. Your tile dealer will have information on these products.

Another solution would be to remove the top layer of grout and replace it with a dark grout that your tile dealer recommends will hold its color. There are special grout-removing tools available at tile dealers. You will need to remove a good quarter-inch to half-inch of the existing grout before regrouting.

Q: The home we purchased has glazed ceramic tile on the walls and floor of a large shower in the master bedroom. The floor surface is dangerously slippery when wet. Is there anything we can do to make it less slippery and safer? -- M.L.

A: Your easiest solution might be to apply peel-and-stick appliques to the surface. The vinyl appliques, available at department and discount stores, have a nonskid finish that give your feet some grip. However, you may not find these very attractive.

A better, more permanent solution is to have the area treated with a chemical solution that etches the tile and makes it slip resistant. The etching is microscopic and does not mar the look of your tile.

One such product is ''Safe-Stride,'' 7231 Carved Stone, Columbia, Md. 21045, (301) 381-5911. Another product is ''Slip-Guard,'' P.O. Box 836, Lake City, Fla. 32056-0836, (904) 752-8814.

Because they are hazardous, both treatments must be applied by a professional. The manufacturers will recommend qualified tile contractors in your area.

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.