A hollow can result when decay associated with old wounds continues to develop over a long period of time. A hollow forms because the wood that develops after the tree is wounded is usually not decayed. Although it is very doubtful that this cavity filling will STOP the decay process, it can help to strengthen the trunk and provide a base for the developing callus around the wound. Cavity fillings are sometimes also desirable for esthetic reasons. When filling in a hollow, be very careful not to injure the tough compartment wall that separates the hollow and decay from the healthy wood.
Sanitation is extremely important.
The microorganisms that may infect wounds often live on dead wood scattered on the ground. Keep dead wood away from healthy trees.