I am wondering if something like Gorilla Waterproof Patch & Seal Tape would work. It says it is permanent but I think you could remove the tape though it is going to be a pain to get the residue off.
Otherwise petroleum jelly should also work. Put your key inside the petroleum jelly and then insert into keyhole and jiggle it around. Will probably have to do this a few times.
No this is not a sexual joke you sickos!