A rusty screw can be hard to shift. Corrosion clogs its thread and the screw feels welded into its hole, stubbornly refusing to move. If the head of the screw has rusted, too, then the slot or cross where the screwdriver fits can crumble away, making it hard to gain a grip. Worse, if you exert too much pressure, you can destroy the slot or cross altogether, "stripping" the screw entirely. The best way to free a rusty screw depends on how far it has stuck.
Tighten the stubborn screw a little if possible, using a screwdriver. Slacken the screw off a fraction, then tighten it again. Do this several times. You'll loosen a little more rusted debris from the screw's thread with each turn. Continue until you can remove it completely.
Tap the material around the head of the stuck screw with a small hammer. The vibration may be enough to loosen a little of the rusted debris clogging the thread, and so free the screw. Try again to turn the screw with a screwdriver.
Scrape any rust away from the edge of the screw head, using the blade of a sharp knife. Wipe up the debris with a rag. Put three to five drops of penetrating oil onto the head of the screw and work the oil around the edge of the screw head, using the rag.
Leave the oil to penetrate around the screw for three to four hours, or if possible, overnight. Then try again to turn the screw using a screwdriver. If there's no movement at all, use an impact driver.
Set the impact driver--a heavy-duty screwdriver that you strike to make it turn--to "unscrew," or to turn counter-clockwise. An impact driver is unsuitable if the screw is embedded in a material such as ceramic or cast iron, that may shatter when you strike the tool. If this is the case, proceed to drilling out the screw.
Place the head of the impact driver in the cross or slot of the rusted screw
Hold the impact driver in place with one hand and strike the other end of the tool smartly with a large hammer. The impact driver converts the force of your blow into torque, or twisting force. This should be enough to free a badly rusted screw. If not, proceed to drilling out the screw.
Use a drill and a metal bit to sheer off chunks from the head of the stubborn screw until you've removed the entire screw head. Typically, this leaves a stub of metal from the screw's shaft sticking out of the material in which the screw is embedded.
Grip the stub of the protruding screw in the jaws of a pair of pliers. Twist the screw stub counter-clockwise to remove it. If there is no stub of screw sticking out, proceed to the next step.
Drill into the screw's shaft using a bit for metal that's smaller in diameter than the screw shaft itself. As you drill deeper into the shaft, the screw will loosen and come out easily, or perhaps break into pieces. In either case, work an iron nail into the hole you've drilled and use this to lever out the screw or its fragments.
Ensure that you're using the correct size of screwdriver for the job. The tip of your screwdriver should fit snugly into the slot or crosscut in the head of the screw. If it doesn't, then the screwdriver will slip. A screwdriver that's too small won't generate enough turning force, or torque, to shift the screw. Try heating the head of a stubborn screw with the tip of a very hot soldering iron, then cool the screw head immediately by applying an ice cube. Do this several times. The expansion and contraction of the metal caused by the heating and cooling may be enough to loosen it. "Popular Mechanics" magazine suggests dipping the tip of the screwdriver in a gritty, household abrasive cleaning compound, as this will give it more grip.