This is his way of marking his territory. It must mean another cat has been there and he is trying to override his scent. You must first wash the door area thoroughly with a disinfectant (2 to 3 times). A cat's sense of smell is way superior to ours, so what you smell as gone may still smell to him.
If your cat is not sterilised, please have that done at your vet as soon as possible. Once sterilised, his likelihood of spraying is reduced. Even if he still does it out of habit, the smell will be less pungent. You should also try spraying vinegar on the door area.