Q: We had floors in our apartment sanded and coated with polyurethane. They did a poor job. Is there a wax or other finish we can apply over the polyurethane that will improve its rough appearance and make it last longer?

A: Wax will help make any finish last longer, including ployurethane, but it will not make a rough finish smooth. This can only be corrected by additional sanding and by applying an extra cost of the same finish.

Q: I have come across some areas on my frame house where the wood seems soft and powdery - whole pieces - while other pieces nearby are not affected at all.Is this dry rot, and if so, what can be done about it?

A: Though I can't say for sure, it sounds to me as though that is what you have - wood rot. It could, however, be termites or carpenter ants, so the first thing to do is call an exterminator to check on this. If his findings are negative and it is rot, pieces that are rotten should be cut out as soon as possible and replaced with sound lumber.

Q: My house dates from 1915 and several of the cast iron radiators have cracked. Can these be repaired?

A: Yes, with epoxy putty or patching compound. Be sure to clean the metal parts next to the crack, and try to scrape rust and dirt out of the crack. Also, be sure the radiator is dry by draining it first. Mix the epoxy according to directions and spread it into and around the crack with a spatula.

Q: The house we bought is about 40 years old and has hand-split cedar shingles on the outside that have never been treated. We now want to apply a clear stain or preservative to help extend their life, but have been told that this would make them turn black and further weaken them. Is this true?

A: Only if you use linseed oil or, as some used to do years ago, crude oil. If you use a prepared clear preservative made by reputable paint or stain manufacturet, it will only darken them slightly, and will definitely help preserve them.

Q: The shower stall and bathroom floor of my upstairs bathroom are covered with ceramic tile. We continually have trouble with mildew and its odor; the grout becomes black and ugly, so we have used straight liquid bleach to clean it. The shower is dried after each use. What else can we do?

A: Sounds to me as though your bathroom needs more ventilation. I recommend installing an exhaust fan and turning it on each time the shower or tub is used.

Q: We recently purchased a butcher block table and were told a rub cooking oil into it every month to keep the wood from drying out. The table top now has a separation at one end and we have been informed that cooking oil is no good for the buter block because it becomes rancid. Can you tell us the best way to car for a butcher block, and is there anything we can do about the "rack in the wood?

A: Cooking oil is likely to become rancid, and I don't advise it for preserving a butcher block top. You should now scrub the surface thoroughly to remove as much of the oil as possible, and sand the top to get rid of the surface that has absorbed the oil. Then treat with at least two coats of a clear penetrating wood sealer, such as one containing tung oil.

As far as the crack is concerned, I'm afraid there is not too much you can do other than fill it in with a wood plastic. These come in colors to match various woods (select the color after the wood has been treated with sealer), but the patch will still show to some extent in most cases.

Questions about home repair should be addressed to Home Improvement Department, the New York Times, 229 W. 43d St., New york, N.Y. 10036. Questions of general interest will be answered in this column, but unpublished letters will not be answered individually.