DEAR SAM: My recently purchased home has a roof with white asphalt shingles. Unfortunately, the galvanized gutters have rusted. Where the downspout from the upper roof empties onto the shingles of the carport roof, an ugly rust stain has formed. Can you suggest some means of eliminating the stain?

ANSWER: If the rust stain were on concrete, I would recommend hydrochloric acid; however, I would not suggest that you try it on the roof shingles. Instead, you should substitute new white shingles directly over the damaged ones.

You may obtain a bundle of shingles. The three tabs should be cut at the notches into individual "shingles." Since the existing nails may not allow you to slide the complete' shingle underneath the upper tab, you can trim the tops (non-granule section) with a shear or knife to miss the nails. When you have completed the placement of the new shingles for the area and nailed them as required, place a dab of roofing cement under each tab so that each shingle will be cemented to the one below (or above).

You could try another method of salvaging the existing by scrapping off the rusty granules. Obtain some white granules by scraping some new white shingles. Brush some roofing cement on the bare old shingles and sprinkle the while granules over the cement. When the cement dries, the granules should stick fast. I believe the first method is preferable.

Don't forget to replace the rusty section of galvanized gutter. Incidentally, if other gutters are of the same vintage, you might still salvage them by giving them a coat of acrylic exterior latex paint, primer and finish. If they are rusty, it is better to replace them, but pain them, as above noted.

DEAR SAM: I live in an almost new, California-style house (over crawl space) with four large sliding glass doors that are not Thermopane. Condensation enters from the glass doors into the metal tracks and seeps into the wall carpeting abutting the tracks. I'm afraid that the carpet will start to mildew and rot. How can I stop the condensation?

ANSWER: In view of the energy crisis, it was poor judgment on the part of the builder to economize on the sliding doors. Duel glazing on the doors would have prevented your problem.

Warm air from the inside is coming in contact with the cold expanse of glass. Since you may not be opening these doors during the wintertime, you could stretch some thin plastic vinyl (polyethylene) over the full opening of the door frame and tack it down tightly with quarter-inch wood strips along the entire perimeter. Lower the thermostat at night for minimal heat loss.

If you already have door drapes, these, too, will add to the insulation necessary for minimizing the condensation.