Is it bad to spray WD-40 in a household door lock. The part that you insert the key in
There is no harm in spraying WD40 into a door lock. We often did this at my old house as the lock would rust from time to time and seize up. We started giving it a dose of WD40 every year and it was fine after that.
I do this evert so often when the lock starts to give me problems. But I don't spray directly into the lock, I spray a key then move the the key in and out of the lock a few times, repeat if necessary