A Phillips screw is a fastener that contains helical ridges wrapped around a long, thin cylinder. Since screws are metal, exposure to weather elements causes them to eventually corrode. As installed screws corrode and rust, they become more difficult to budge. Attempting to remove rusted screws with power tools only increases the risk of stripping the screws. Once screws are stripped, they are basically useless. Fortunately, certain supplies and techniques are effective at removing rusty screws without damaging them.
Spray the rusted screw head and shank with penetrating oil. Allow the screw to absorb the oil for a couple of minutes.
Choose a new screwdriver that precisely fits the screw head. Do not use a partly used or well-used screwdriver.
Dip the end of the screwdriver bit into valve lapping compound. Use the coarse grit valve lapping compound, not the fine grit.
Insert the screwdriver tip into the screw head groove. Tap the screwdriver downward with a hammer to break up the corrosion.
Turn the screwdriver counterclockwise, while applying pressure to the screwdriver. Spray more penetrating oil onto the screw.
Continue turning and applying pressure to the screwdriver until the screw is completely removed. Soak the rusted screw in a container of vinegar for 24 hours.
If the rusted screw will not budge, remove it with a powered screw extractor.