All sinks have a locking nut on the underside that holds the faucet in place. Exposure to moisture often results in rust and mineral deposits around the nut. Over time, such deposits create an adhesive bond. There are several options when it comes to removing a stuck faucet nut, depending on the severity of the rust problem.
Place an appropriately sized wrench on the nut and attempt to rotate it in either direction. Even if the nut can only be tightened initially, it may be enough to break through the rust, which will allow the nut to be loosened.
Place a centre punch on a stable spot on the nut. Tap the punch with a hammer to loosen the bond to the pipe. Attempt to loosen the nut with the wrench again. A flathead screwdriver can be used in place of a punch if necessary.
Turn on a hair dryer at high heat and point it at the nut for five minutes. The heat can cause softening and expansion of the metal, which may help with nut removal. Attempt to loosen the nut with the wrench.
Apply a penetrating oil to the nut. Attempt to get the oil inside the threads for best results. Let the oil soak in for a minimum of one hour and loosen the nut with the wrench. The longer you allow the oil to work, the better. Apply the oil over the course of 24 hours, if necessary.