Miracle sprays and magic tapes fix the demo, not your house

These get sold to you in three places: a late-night demo where the surface is clean, dry and perfectly lit, the checkout-aisle endcap built to catch a tired man on a Saturday, and the shelf at the hardware store labeled "as seen on." A demo is a controlled environment. Your basement isn't. Wanting the shortcut doesn't make you foolish, because everybody does. But a shortcut that hides the problem costs you the repair twice.

Spray-on rubber sealant for leaks

What it promises: Spray it on and the leak is over, whether it's a pipe, a gutter or a roof.

Why skip it: Coatings need a clean, dry surface to grip, and a leaking pipe is neither. On a pressurized line the water keeps pushing at the coating from underneath. On a roof, you cover the spot where water gets in and leave the actual failure, usually flashing, to keep working underneath. When you finally do the real repair, you scrape off the goo first.

Do this instead: Shut off the water at the fixture stop or the main, then open a low faucet to drain the line. Cut out the bad section and fit a repair coupling. Push-to-connect couplings (SharkBite is the well-known brand) are sold at any hardware store for common pipe types. Check what your pipe is before you buy one. A leak inside a wall, on the main line, or on pipe you can't identify is a licensed plumber's job. Codes vary by area, and some places require a licensed plumber for certain work, so call your local building department if you're unsure. For a roof leak, have a roofer find the entry point.

Worth it when: The job isn't under pressure and doesn't matter much, like a cracked plastic bin or a gutter seam you'll replace in spring. Read the can for surface prep and cure time, and follow both.

The one can that claims to do everything

What it promises: Fixes squeaks, sticks, rust and whatever else you've got.

Why skip it: The famous blue-and-yellow can is a water displacer and solvent with a light lubricant in it. That's what the name stands for. It's good at some jobs and mediocre at others, and plenty of door and opener makers specify a different lube in their manuals. Spraying it on everything is how a squeak comes back in a week.

Do this instead: Read the manual for the thing that squeaks. For most hinges and rollers, a silicone spray or white lithium grease is the plain answer. Never spray any aerosol into an electrical panel or near a pilot light or open flame. Cans are flammable.

Worth it when: You're freeing a rusted bolt, driving moisture out of a wet connector, or lifting sticky label residue. That's the can's real job.

Magic waterproof tape on a pipe

What it promises: Wrap it around a leak and walk away.

Why skip it: A pipe usually leaks because of corrosion or a failed joint. The metal around the pinhole is thin too, so tape on a pressurized line buys you a bigger leak, later, at a worse time.

Do this instead: Replace the section with a coupling, as above. For threaded joints, the right tape is cheap and already exists: PTFE thread tape, a few wraps on the male threads, in the direction the fitting tightens. Gas lines are a different matter. Yellow gas-rated tape exists, but gas work belongs to a licensed pro in most places. Shut off the gas, leave the house if you smell it, and call the utility from outside.

Worth it when: You need an emergency splint on a garden hose or a coolant hose to get home. Self-fusing silicone tape, stretched and overlapped, is a legitimate temporary fix. Replace the hose the next day.

Drain gels and "dissolves anything" openers

What it promises: Pour it in, go make coffee, come back to a clear drain.

Why skip it: The product sits in standing water above a clog it can't fully reach. Now you've got caustic liquid in a pipe you're about to plunge or open, and that splashes into your eyes. Mixing two products is worse.

Do this instead: A plunger, then a hand auger. Unclog a Drain With a Plunger, a Snake and Ten Minutes walks through both in order.

Worth it when: The snake fails and the plumber's a day away. Use it once, follow the label for what pipes it's rated for, wear eye protection and gloves, and tell the plumber what went down the drain.

Glue that bonds anything

What it promises: Wood to metal to plastic, instantly, forever.

Why skip it: A wobbly chair joint needs a tight fit, glue spread on both surfaces, and clamps holding it overnight. Instant glue is brittle, doesn't fill gaps and doesn't flex. Worse, once it's cured in a joint, old glue and finish block a proper repair later.

Do this instead: Clean out the old glue, use wood glue, clamp it and leave it alone until morning. Reglue a Wobbly Chair So It Outlasts the Rest of the Set has the steps.

Worth it when: The job is small, light and non-structural: a loose rubber foot, a cracked knob, a snapped bit of trim. Keep it off your fingers.

Engine-in-a-can additives

What it promises: A mower or snowblower that won't start comes back to life with one bottle.

Why skip it: A small engine that won't run usually has stale fuel, a fouled plug, a dirty air filter or a gummed carburetor. A bottle doesn't clean a clogged jet. You still have to open it up.

Do this instead: Drain the old gas, put in fresh, check the plug, clean the filter. Your Mower Won't Start. Check Spark, Fuel, Air In That Order covers the sequence. Turn the engine off and pull the plug wire before you work near the blade.

Worth it when: You're adding fuel stabilizer to fresh gas before storing equipment for the season. Follow the label's ratio.

How to say no at the counter

Three lines that work, and none of them is rude:

• "I'm good, I'll just take the coupling."
• "Does the label say what surface it's rated for?"
• "Can you show me where it says it works on wet pipe?"

If the instructions require a clean, dry surface and your problem is wet and corroded, the can just answered for you.