Rust is a natural occurrence on many metal products caused by the combination of oxygen and moisture. It is typical for older cars or those that have sustained water damage to have rusted number plate bolts, making it difficult to replace or remove the number plate.
Spray the rusted bolt liberally with a penetrating solvent such as Liquid Wrench or Rust Busters. Let the solvent sit for five minutes or according to specific package instructions.
Use the socket wrench to begin removing the bolt. Make sure your socket attachment is the correct size for the bolt. Slowly work the bolt until it begins to budge, then remove the bolt. If the socket wrench begins to slip, stop immediately. Use a hammer to help you turn the socket wrench if it is stiff.
Break the bolt off if you are unable to remove it. Wedge a flathead screwdriver between the bolt head and the number plate. Position the screwdriver facing down if the plate position allows it. Be careful not to scratch the paint. Carefully keep the screwdriver wedged and hammer the handle. If the bolt was too rusted to loosen, it will likely break off fairly easily.
Not everyone has an impact wrench, but, if you do, use it in place of the socket wrench when trying to remove the bolt. Air-powered or electric impact wrenches will provide more torque than you can with your hands, increasing the chance of removing the bolt safely. If you just need to remove the number plate, try pulling it off of the bolts. Aluminium is used to make number plates; it tends to be softer and more pliable than tough bolts. Work the plate back and forth until the mounting holes stretch and bend around the rusted bolts.