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HANDYMAN ON CALL

Don't try painting over oil-stained concrete

Q: I am trying to get grease and oil stains off my concrete garage floor. Can I paint over the stains? What is best to put on the floor?

PAUL GREELEY, Cotuit

A: No, you cannot paint over the stains; in fact, don't try to paint the oil-stained concrete under any circumstances. While the problem with oil stains on concrete has been answered many times in the past, it's worth mentioning one time this summer. File this answer for future use.

Scrape off any grease or hardened oil that can be removed with a metal scraper. Then apply paint thinner generously to the stains, which will dissolve, we hope, the oil. Then sprinkle, also generously, an absorbent clay such as cat litter or Speedy-Dry on the stain. Let it stand for an hour, then sweep it up and throw it away. Repeat as necessary.

Once you get everything clean and dry, stain the floor with a semitransparent deck stain; over bare concrete, it will not peel and will look pretty good for up to five years. Or, you can buy liners for garage floors; it sounds rather inane to me, but the Improvements Catalog sells PVC liners of different sizes, and an under-the-car mat. They are pricey, a sight more than a gallon of stain. The phone number is 800-642-2112.

Q: I have had a gas-fired water boiler since 1962. I plan to replace it with a new, high-efficiency unit. The chimney has a clay liner; do I need a stainless steel liner?

WORRIED

A: You may need a stainless steel or special aluminum alloy liner. I believe that any installation of a gas-fired unit requires a new liner. Check with your local building department or fire marshal. The reason for strict regulations: Burning gas creates a great deal of water vapor, which combines with contaminants in the chimney to form an acid that can deteriorate a clay liner quickly. Your present clay liner has been handling gas fumes for 43 years, a very long time.

One answer to this dilemma is to install a power-vented unit, which exhausts low-temperature fumes through the wall, bypassing the chimney altogether.

Q: The landscape ties of my retaining wall have split and rotted. Can I fill those holes, or should I replace those top ties?

ROBERT DORSON, Lynn

A: You might be able to fill the holes, even if they are deep, if the splits are not too disastrous. Cut out any decay. Treat with bleach to kill any fungus that caused the decay in the first place; rinse and let dry. You can fill the voids with an epoxy wood filler; Bondo, used to fix cars, is good enough for this purpose. But you do not want to fill big voids with Bondo because that would be too expensive. So, fill the voids with sand up to an inch or so of the top, then put in the Bondo. Finish off the top with a cap of pressure-treated lumber, a 2-by-10 or 2-by-12 so that it overlaps the wall by an inch or so, providing a proper drip edge.

If that does not appeal to you, replace the top timbers with pressure-treated beams.

Q: I have a nice-looking island counter in my kitchen in Maine, with a maple top. It has quite a few black stains, small but numerous. I was told it is a polyurethane finish, and it looks terrible. How can I clean it, or do I have to refinish it?

CAITLIN DEVAUL, Milton

A: The black stains are typical of a varnished surface exposed to water. You can try washing with Spic and Span and water to which a cup of bleach has been added. If it cleans up, then you can sand lightly and apply two coats of an oil-based, semigloss polyurethane varnish.

If it does not clean up, then sand to the bare wood (use a power sander) and apply three coats of an oil-based polyurethane varnish. Or, if you really do not want more urethane varnish, oil the maple top. Apply a generous coat of mineral oil, wait 15 minutes, then rub with your hands. There will be a lot of excess oil, so wipe it all off, or as much as will wipe off, with a dry cloth.

The next day, repeat this procedure, or repeat it as often as you like or as rarely as you like. An oiled finish is soft but nice looking, and will resist water. The rule in oiling wood is this: once a day for a week, once a week for a month, once a month for a year, and once a year forever.

Q: The radiant heating system in the concrete-slab floor in my house has been disconnected, but remaining domestic water pipes in the slab keep freezing in winter because there is no heat there. They have not yet burst, but I think it is only a matter of time that they will. It is impractical and expensive to relocate the domestic pipes. Will insulating the foundation (6 inches are exposed) stop the freezing. Is it worth it?

O.J.B., Chelmsford

A: It is worth it to try because it is not that difficult and is a lot cheaper than relocating the pipes. Buy 2-inch-thick rigid foam called High R Sheathing or Thermax (two brand names), and glue it to the foundation, from just under the siding and down at least 12 inches into the ground; 24 inches would be better. Cover this with a protective covering such as 1/2-inch cement board. Put flashing between the insulation and covering and the siding, so no water will get into the insulation. If you cannot find 2-inch insulation, put on two layers of 1-inch insulation.

This technique was used in the 1970s during the first energy crisis, but went out of favor when oil prices dropped.

Q: Wasps are making themselves a comfy home behind the shutters of one of my bedroom windows. I don't like to use Raid or any other insecticide. How can I get rid of them?

WILD OVER WASPS

A: Get rid of them? Wait for cold weather, when they will die off. If they are not bothering you (you have good screens in your windows?), then leave them alone. They are beneficial insects, eating bad bugs and doing some pollinating too, I believe.

Q: I installed an oak floor 20 years ago, finished and waxed to look like weathered planks, fastened with iron nails. I spilled water on it, resulting in blotches, some light, some dark. I tried a wood cleaner (Treewax) and the blotches are worse. What can I do? I cannot sand because the iron nails will tear up the sandpaper. The boards are also ridged, as if they have eroded in the weather.

FIORE RULLO, Harwich

A: That is a dilemma. I think the blotches are in the wax, not the finish, so the wax must be removed with paint thinner.

If the blotches fade, you can re-wax. I suggest you use a self-polishing wax designed for wood.

A last resort is to pull the nails and sand the finish to the bare wood, apply a light-gray stain to simulate weathering, and apply three coats of a semigloss, oil-based polyurethane varnish. And no wax.

Globe Handyman on Call Peter Hotton is available 1-6 p.m. Tuesdays to answer questions. Call 617-929-2930. Hotton also chats online 2-3 p.m. Thursdays. Go to Boston.com. Hotton's e-mail is photton@globe.com.

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