To properly treat this wound, I'll use a knife to reopen the area back to green cambium. The sharp edge of the knife will remove all the decaying and minimize the tearing of tissue during cutting. The knife I use here is a grafting knife that is very sharp. You don't want to use your only grafting knife for this work. It would be spoiled.
Once the tree has been watered and the wound has dried just a little I will cover it with sealant.