The recipe to loosen a rusted screw is a relatively simple one. To dissolve the corrosive seal, use a penetrating oil spray, such as WD-40®. Spray the screw head and allow one or two minutes for the lubricant to penetrate the corrosion around the screw. As an alternative, you can try brushing kerosene on it. At this point, avoid using a power tool to remove the screw, since the potential to strip the head is too great. Instead, choose a screwdriver with an excellent fit for the head of the screw.
If a flathead is required, be sure the tip fills the slotted head of the rusted screw securely and is not too thin for the slot or too narrow for the screw head. A Phillips screwdriver head should likewise provide good bite into the head. If the tool tip shifts position within the screw’s head, it's too small. If it doesn’t fit securely into the head, it's too large.
With the correct screwdriver inserted, apply significant pressure by leaning into the screw. This keeps the screwdriver from slipping and possibly stripping the head, and it also creates better bite. Attempt to loosen the rusted screw by turning it counter-clockwise. It may take a moment or two of constant torque to crack the seal and get the screw moving.
If the first attempt does not work, apply more lubricant and try gently tapping the screw’s head from all angles. You might also insert your screwdriver tip and use a hammer to lightly tap downwards a few times. This can help to break the corrosive seal. Try turning the screw again.
If the head of the screw becomes stripped or damaged, or it simply won’t budge, you may need to resort to a powered screw extractor. There are several models, but in essence, these tools drill into the screw itself counter-clockwise, using a reverse thread. Note that some screw extractors have the potential to ruin the threads of the screw hole, requiring it to be re-tapped afterwards. Screw extractors are a last resort when manual removal has failed.
Screw extractors and lubricants are available at hardware stores everywhere. When a rusted screw is removed, it is best to replace it with a new one, even if the original screw is otherwise undamaged.