Try WD40. When we have screws that are rusted up, we squirt that on the screw and nut. Let it soak overnight, next day with a little pressure, boom it comes off. When my key for my car starts to stick in the door to unlock, I squirt that on the key and in the keyhole and it works like brand new. So squirt that on the knobs that are rusted up. It should help alot
Kim