Given the assumption of corroded metal nuts, applying a penetrating oil is a critical step. From your position under the sink, aim the nozzle of the penetrating oil spray can as best you can towards the threads of the faucet tailpiece, just above the mounting nuts. Apply a liberal amount of oil, attempting to coat the nuts and threads completely. Allow the oil to sit and work its way into the corroded threads for at least 15-20 minutes, or longer for severely seized nuts. This chemical action will do much of the hard work for you, making the nuts significantly easier to break free.
Step 2/6
Apply Penetrating Oil2 min