It does seem counter-intuitive to leave a wound uncovered when we consider it from a human point of view. Many people treat tree wounds just like they would treat cuts in the human body--with a dressing. That is the way we tend to think about wounds, but remember a tree is not a human, or even an animal, and responds in a different manner.

The basic idea is that trees seal off injuries; they do not heal like we do. If there is damage they build a wall of tissue around it and carry on growing. When you apply tar, or aloe gel or paint to a tree wound you are sealing in moisture and preventing the wall of tissue from forming. Some of the ingredients can actually serve as a food source for pathogens and none of them will stop rot.

Purchasing a special tree wound dressing is a waste of money. Science has established for well over 50 years that wound dressings and pruning paints do not prevent wood rot. So it's about time we relegated this practice to the dust bin of history.

The best treatment of a pruning cut is no treatment at all. Trees have been around for 360 million years and managed quite well without our "help." If you see a tree with a fresh pruning cut, chances are it needs a drink of water more than anything else.  

This tree is responding to a proper pruning cut, by forming a round doughnut of callus tissue which will eventually close the wound.

Black paint applied to these wounds will probably do more harm to the tree than good. 

For more information on the side effects and dangers of Pruning Paint.