Q: The exposed basement walls of our newly constructed house are made of concrete block covered with parging (a troweled/on layer of cement). During the last winter numerous whitish hairline cracks have formed in the wall, and scrubbing with a wire brush does not remove the white stains. Our builder has applied another coat of the parging, but the same defects reappeared and he maintains that there in nothing further he can do. Is there any sort of permanent coating that we can apply that would cover the cracks and prevent the white stains from bleeding through?

A: The cracks are due to a slight deficiency in the way the parge coat of mortar was mixed, or the way it was applied. The white stains sound to me like efflorescence -- a fairly common problem with concrete and masonry walls. It is caused by moisture that leaches out salts created by excess alkali in the masonry. You can remove these white stains by washing with a dilute solution of muriatic acid (sold in paint and hardware stores), then rinsing with water. Wear rubber gloves to protect your hands, and mix the acid and water in a plastic bucket.

After this the cracks can be filled by using a vinyl concrete patching cement. If they are too fine for this, then chances are they would be filled by painting the entire wall with a heavybodied latex masonry paint, the kind made especially for use on basement walls.

Q: I used duct tape to apply plastic to the outside of my windows, but the plastic tore, so I had to remove it all. There is still a sticky residue from the tape on the white aluminum frames of my windows. I nail polish remover, but none of these work. What do you suggest for removing the sticky stuff?

A: As a rule, it can be removed with a lacquer thinner. Nail polish remover is basically lacquer thinner and should have worked. I suspect you did not use enough or did not give it time to work. Buy some lacquer thinner and saturate a small cloth with it. Wipe on liberally, wait about 30 seconds, then wet the cloth again with the thinner and rub vigorously over the same palce. There is one possible problem -- this may also remove the white paint if your frames are painted. If so, try using rubber cement thinner (sold in art supply stores) instead of lacquer thinner.

Q: About 20 years ago the ceramic tile in my bathroom was painted, along with the walls, with what appears to be semigloss paint. Now signs of wear are starting to show signs in small areas under the soap receptacle and above the sink. How can the paint be removed from all the tile?

A: Any paint that has stood up for 20 years has done very well, so I am not sure you want to remove it instead of merely touching up the worn spots. At any rate, if you want to get it all off, you can do so with semipast paint remover. I recommend one of the waterwash types because this will be easier to work with.

Q: Our brick house has green moss growing over a considerable part of the outside, mostly on the north side, and it seems to be spreading rapidly. Does this mean that there is moisture in the bricks? What should be done about removing the moss and preventing it from coming back?

A: The moss is more prevalent on the north side because this is usually the side that gets the least sun. Moss seldom thrives where there is adequate sunlight and plenty of ventilation. But because bricks are porous and hold moisture they provide an excellent base for moss to grow. You can kill and remove the moss by scrubbing the bricks with mildewcide (the type sold in swimming pool supply stores). To help keep it from comming back, prune off large overhanging tree branches or shrubs that keep that area in shade, and apply clear sealer to the bricks so they will not absorb as much moisture or stay wet as long.

Q: My house has aluminum siding that was put up about 15 years ago. Is it possible to paint over it?

A: It certainly is. First, clean the dirty areas thoroughly -- especially under overhanging eaves and other places where wind and rain do not often reach. Then sand all glossy sections lightly to kill the shine and apply a coat of exterior primer, followed by a finish coat in the color of your choice. You can use either alkyd-or latex-base paint.

Q: My house has stained cedar shingles on the outside and on the shady side there is a green stain that I think is mildew. I have tried rubbing it off with detergent and ammonia, but this goes very slowly and also takes the color out. What is the best way to solve this problem?

A: To kill meldew you should wash it with a solution of one quart fresh liquid landry bleach and three quarts of water. To this add some powdered detergent (no ammonia). Scrub this mixture on, let it dry, then rinse thoroughly. When you put new stain on (don't do this until you have scrubbed the shingles as described), make sure you use one that is mildew-resistant.

Questions about home repair problems should be addressed to Bernard Gladstone in care of The New York Times Syndication Sales Corp., 200 Park Ave., New York, N.Y. 10017. Questions of general interest will be answered in this column. Unpublished letters cannot be answered individually.