Q: My son has reached the age when the power tools in my shop seem very interesting to him. I don't mind if he uses them under my supervision, but am afraid that he may use them when I'm not home. Is there some easy way to prevent this? I can lock my portable power tools in a cabinet. It's my larger tools that concern me. A: When I was young, my father had his whole shop wired through a single fuse box with a master switch he could lock in the off position. A simpler and less expensive solution is to put a little padlock on the plug of any tool you wish to "kidproof," sliding it through the hole in one of the plug's tabs. Most modern power tools, by the way, now have safety locks built into them. Q: Could you explain the difference between a heat pump and a heat exchanger? A: A heat pump is essentially an air-conditioner that can be run in reverse. It uses a powerful motor and a compressor to run a refrigerant such as freon through an evaporation-and-condensation cycle. The refrigerant picks up heat when it evaporates, carries it to another location, then dumps the heat when it condenses.

A heat exchanger is simply a series of closely spaced plates or fins. Cold air flows on one side of the plates, warm air on the other. In the process, heat passes from the warm air through the plates to the cold air. A small electric motor and fan are all that's required to power a heat exchanger.

The two devices do two entirely different jobs. The heat pump can move heat into your home (to heat it) or out of your home (to cool it). All a heat exchanger can do is help to maintain your indoor temperature when you bring in fresh air for ventilation. Q: I took my front door off its hinges to give it a coat of paint. When I did, three pieces of cardboard fell out from behind the hinges. After painting, I put the door back, with a piece of cardboard behind each hinge. The door rubbed against the frame near the top of the side away from the hinges, and also along the bottom edge, near the corner away from the hinges.

I took the cardboard out, but the door still rubs. Could the thickness of the coat of paint make the door rub? Will I have to shave the door and repaint? A: I don't think the paint is your problem. My guess is that you put the cardboard shims back in the wrong place. To correct the rubbing pattern you describe, put two of the shims behind the bottom hinge, and one behind the middle hinge. Your door is most likely hanging at an angle, and putting the shims back should solve the problem. If not, try three shims behind the bottom hinge and two behind the middle one. Also, make sure the screws securing the top hinge are drawn up tight. If they're stripped, glue some toothpicks in the screw holes with a fast-setting epoxy. Let it set for 10 minutes, then replace the screws. Q: I have a dehumidifier in my basement with a plastic bucket to catch the water. The bucket fills up every few days, and emptying it is a bother. Is there an easy way to hook up a hose or something to drain the water automatically? A: Take a close look at the bucket. Some brands have a threaded nipple molded in to accept a garden hose. You can then knock out the plug that seals off this nipple and the bucket will drain through the hose. Run the hose outdoors or to a drain. You may have to place your dehumidifier up on a table or other surface to get it high enough so gravity can drain the bucket.

If your bucket has no nipple, you can improvise: Buy some rubber or neoprene tubing long enough to run from the bucket to a drain or to the outdoors. Drill a snug hole for the tubing through the side of the bucket near the bottom. Slip the tubing through the hole about an inch and cement it in place with silicone rubber caulk, both in and outside the bucket. Q: I have solid walnut salad bowls that once were beautiful. Now they are scratched and ugly. I'm prepared to sand them down, but have no idea what to use as a finish that would be safe for food. Can you help me? A: The only product I know of is Behlen Salad Bowl Finish, which is FDA-approved. You can buy it from Woodcraft Supply, 313 Montvale Avenue, Woburn, Maine 01801. Price is $5.40 postpaid. Q: I tried doing some veneer repairs with a latex base contact cement, but had a devil of a time. The cement made the veneer curl up and didn't seem to stick very well. Is contact cement the wrong type of adhesive? A: No, contact cement will work. But don't use the latex type. The water in it causes the veneer to curl. Use a solvent-based cement instead. There are two types, one flammable, the other not. For safety, use the nonflammable type. Still, provide plenty of ventilation. The fumes won't burn, but they shouldn't be breathed any more than necessary.